1. What happened
During importing 'TextMesh Pro' Asset Store package I'm getting the following errors:

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Scripting.ScriptCompilation.EditorBuildRules.GetAllScriptAssemblies (IEnumerable`1 allSourceFiles, System.String projectDirectory, BuildFlags buildFlags, UnityEditor.Scripting.ScriptCompilation.ScriptAssemblySettings settings, UnityEditor.Scripting.ScriptCompilation.CompilationAssemblies assemblies) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Scripting/ScriptCompilation/EditorBuildRules.cs:180)
UnityEditor.Scripting.ScriptCompilation.EditorCompilation.GetAllScriptAssemblies (BuildFlags buildFlags, EditorScriptCompilationOptions options,
....

and

NullReferenceException: Object reference not set to an instance of an object
TMPro.TMP_Settings.get_defaultFontAsset ()
TMPro.TextMeshProUGUI.LoadFontAsset ()
TMPro.TextMeshProUGUI.OnValidate ()

2. How we can reproduce it using the example you attached
- Have a new project
- Find the package in Asset Store (https://www.assetstore.unity3d.com/en/#!/content/84126)
- Import
- It asks for script updater to be run. Confirm.
---> Errors appear in Console at import completion

Note that there was only the second error in 5.6.0p1. And there are no errors in 5.5.2p2.