Steps to reproduce:

1) Download attached project or create a new one and open in Unity
2) Go to Project >> Packages >> Manage
3) Click on Install module and add Textmeshpro
4) In the Project tab, R.click >> Create >> TextMeshPro >> Sprite Asset
Note: Warning will appear in the Console tab : A texture which contains sprites must first be selected in order to create a TextMesh Pro Sprite Asset.(which is correct)
5) Go to Project >> Packages >> Manage and remove Textmeshpro package
Note:
error CS2001: Source file `Packages/com.unity.textmeshpro/Scripts/Editor/DropdownOptionListDrawer.cs' could not be found
error CS2001: Source file `Packages/com.unity.textmeshpro/Scripts/Editor/EditorCoroutine.cs' could not be found
error CS2001: Source file `Packages/com.unity.textmeshpro/Scripts/Editor/GlyphInfoDrawer.cs' could not be found
error CS2001: Source file `Packages/com.unity.textmeshpro/Scripts/Editor/KerningPairDrawer.cs' could not be found
errors will appear, that scripts could not be found. But none of the objects was created with Textmeshpro (check attached image "errors.png")

Workaround: relaunch the Editor or add any other new package

Reproduced with:
2018.1.0

Note: This package UI was introduced in 2018.1.0b3