How to reproduce:
1. One a new Unity project
2. Create a Cube in the Scene
3. From Project Browser, drag and drop any texture from "Packages/ TextMesh Pro/ Editor Resources/ Gizmos" onto the Cube
4. Save the Scene
5. Restart Unity Editor

Expected result: The Cube still has the material that was generated from the texture.
Actual result: The Cube has no (missing) material.

Reproducible with - 2018.2.0b1, 2018.2.0f2, 2018.3.0a4

Notes:
- Could not be tested on versions prior to 2018.2, since the above mentioned "Packages" folder was introduced in 2018.2.
- After step 3 is performed (after drag and dropping), a warning and an error appear in the Console:
- Error: "Read only asset Packages/com.unity.textmeshpro/Editor Resources/Gizmos/Materials has no meta file."
- Warning: "Ignoring asset refresh of Packages/com.unity.textmeshpro/Editor Resources/Gizmos/Materials/TextMeshPro Inline Graphics icon.mat because the folder it is in has not been imported yet!"
- Materials are created, but they are not accessible from the Project Browser and they are not referenced after restarting Unity.

Update: After the fix, drag & dropping packaged textures onto scene objects is disabled.

Fixed in 2019.1.0a3
Backported to 2018.2.11f1