Steps To Reproduce:

1. Create a project.
2. Window > Text Mesh Pro > Import TMP Examples and Extras.
3. "Import Unity package" window will open, select Import.
4. "Resource Importer" window will open, select Import TMP Essentials.

Reproducible On : 2018.3.0a9, 2018.3.0a6

Not Reproducible On : 2018.3.0a5

Package Manager Version: 1.2.4

Stack Trace:
MissingReferenceException: The object of type 'TextMeshPro' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
UnityEngine.Component.GetComponent[T] () (at /Users/builduser/buildslave/unity/build/Runtime/Export/Component.bindings.cs:42)
TMPro.TextMeshPro.Awake () (at <e401af8928bb431ea80daa802b80ff47>:0)
TMPro.TextMeshPro.ON_RESOURCES_LOADED () (at <e401af8928bb431ea80daa802b80ff47>:0)
TMPro.FastAction.Call () (at <e401af8928bb431ea80daa802b80ff47>:0)
TMPro.TMPro_EventManager.ON_RESOURCES_LOADED () (at <e401af8928bb431ea80daa802b80ff47>:0)
TMPro.PackageResourceImporterWindow.ImportCallback (System.String packageName) (at <e401af8928bb431ea80daa802b80ff47>:0)
UnityEditor.AssetDatabase.Internal_CallImportPackageCompleted (System.String packageName) (at /Users/builduser/buildslave/unity/build/Editor/Mono/AssetDatabase/AssetDatabase.cs:36)

Environment: Occurs on Win and Mac.