Painting in the Tile Palette window throws NullReferenceException, after deleting Layer1 child GameObject (with Tilemap and Tilemap Renderer components), from Tile Palette prefab

Note:
Refer attached video

Steps to reproduce:
1. Open Unity > Create New Project
2. Window > 2D > Tile Palette
3. Create New Palette > Create New Palette
4. Select "Paint with active brush" tool or press B.
5. Open New Palette in Prefab mode
6. Delete Layer1 GameObject with Tilemap and Tilemap Renderer components
7. Save changes and exit prefab mode
8. Click on Tile Palette window to paint

Actual result:
Painting in Tile Palette window, after deleting child GameObject with Tilemap component, or removing Tilemap component, throws NullReferenceException

Expected result:
Tile Palette window should remain empty if "Active Tilemap" is "Nothing" and disabled.

Reproduced in:
2019.2.0a2, 2019.1.0b3, 2019.1.0a15, 2018.3.0b1, 2018.2.0a1, 2017.4.15f1, 2017.2.5f1, 2017.2.0b1, 2017.2.0a4

Environment:
Windows & Mac

Stacktrace:
Refer attached file