使用一对多关联模型时 ，当没有查询到数据时要怎么处理才不会报错

代码块

报错信息

{
    "message": "Trying to get property 'teacher' of non-object",
    "exception": "ErrorException",
    "file": "/home/vagrant/xes/app/Http/Controllers/Api/CourseController.php",
    "line": 13,
    "trace": [
        {
            "file": "/home/vagrant/xes/app/Http/Controllers/Api/CourseController.php",
            "line": 13,
            "function": "handleError",
            "class": "Illuminate\\Foundation\\Bootstrap\\HandleExceptions",
            "type": "->"
        },
        {
            "function": "index",
            "class": "App\\Http\\Controllers\\Api\\CourseController",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Routing/Controller.php",
            "line": 54,
            "function": "call_user_func_array"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Routing/ControllerDispatcher.php",
            "line": 45,
            "function": "callAction",
            "class": "Illuminate\\Routing\\Controller",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Routing/Route.php",
            "line": 219,
            "function": "dispatch",
            "class": "Illuminate\\Routing\\ControllerDispatcher",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Routing/Route.php",
            "line": 176,
            "function": "runController",
            "class": "Illuminate\\Routing\\Route",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Routing/Router.php",
            "line": 681,
            "function": "run",
            "class": "Illuminate\\Routing\\Route",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 130,
            "function": "Illuminate\\Routing\\{closure}",
            "class": "Illuminate\\Routing\\Router",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/app/Http/Middleware/Api/RefreshTokenMiddleware.php",
            "line": 30,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 171,
            "function": "handle",
            "class": "App\\Http\\Middleware\\Api\\RefreshTokenMiddleware",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/medz/cors/src/Laravel/Middleware/ShouldGroup.php",
            "line": 23,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 171,
            "function": "handle",
            "class": "Medz\\Cors\\Laravel\\Middleware\\ShouldGroup",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Routing/Middleware/SubstituteBindings.php",
            "line": 41,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 171,
            "function": "handle",
            "class": "Illuminate\\Routing\\Middleware\\SubstituteBindings",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Routing/Middleware/ThrottleRequests.php",
            "line": 59,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 171,
            "function": "handle",
            "class": "Illuminate\\Routing\\Middleware\\ThrottleRequests",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 105,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Routing/Router.php",
            "line": 683,
            "function": "then",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Routing/Router.php",
            "line": 658,
            "function": "runRouteWithinStack",
            "class": "Illuminate\\Routing\\Router",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Routing/Router.php",
            "line": 624,
            "function": "runRoute",
            "class": "Illuminate\\Routing\\Router",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Routing/Router.php",
            "line": 613,
            "function": "dispatchToRoute",
            "class": "Illuminate\\Routing\\Router",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Foundation/Http/Kernel.php",
            "line": 170,
            "function": "dispatch",
            "class": "Illuminate\\Routing\\Router",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 130,
            "function": "Illuminate\\Foundation\\Http\\{closure}",
            "class": "Illuminate\\Foundation\\Http\\Kernel",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/TransformsRequest.php",
            "line": 21,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 171,
            "function": "handle",
            "class": "Illuminate\\Foundation\\Http\\Middleware\\TransformsRequest",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/TransformsRequest.php",
            "line": 21,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 171,
            "function": "handle",
            "class": "Illuminate\\Foundation\\Http\\Middleware\\TransformsRequest",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/ValidatePostSize.php",
            "line": 27,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 171,
            "function": "handle",
            "class": "Illuminate\\Foundation\\Http\\Middleware\\ValidatePostSize",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/CheckForMaintenanceMode.php",
            "line": 63,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 171,
            "function": "handle",
            "class": "Illuminate\\Foundation\\Http\\Middleware\\CheckForMaintenanceMode",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/fideloper/proxy/src/TrustProxies.php",
            "line": 57,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 171,
            "function": "handle",
            "class": "Fideloper\\Proxy\\TrustProxies",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/medz/cors/src/Laravel/Middleware/Cors.php",
            "line": 62,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 171,
            "function": "handle",
            "class": "Medz\\Cors\\Laravel\\Middleware\\Cors",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php",
            "line": 105,
            "function": "Illuminate\\Pipeline\\{closure}",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Foundation/Http/Kernel.php",
            "line": 145,
            "function": "then",
            "class": "Illuminate\\Pipeline\\Pipeline",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/vendor/laravel/framework/src/Illuminate/Foundation/Http/Kernel.php",
            "line": 110,
            "function": "sendRequestThroughRouter",
            "class": "Illuminate\\Foundation\\Http\\Kernel",
            "type": "->"
        },
        {
            "file": "/home/vagrant/xes/public/index.php",
            "line": 55,
            "function": "handle",
            "class": "Illuminate\\Foundation\\Http\\Kernel",
            "type": "->"
        }
    ]
}

数据库
teacher表

course表