问题描述：

导入资源的Prefab没有预览图，而且当拖拽使用时不可用并且程序报错

问题发生时间：

古迹探险案例---地形设计

报错信息：

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.GameObjectInspector.OnSceneDrag (UnityEditor.SceneView sceneView) (at C:/buildslave/unity/build/Editor/Mono/Inspector/GameObjectInspector.cs:775)
UnityEditor.SceneView.CallEditorDragFunctions () (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:2165)
UnityEditor.SceneView.HandleDragging () (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:2180)
UnityEditor.SceneView.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:1639)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:232)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:285)
UnityEditor.HostView.Invoke (System.String methodName) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:278)
UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:245)