[IMGUI] RGB, R and G tabs are missing in Baked Lightmap Preview Window.

RGB, R and G tabs are missing in Baked Lightmap Preview Window.

Steps to repro:
1. Open the attached project.
2. Select the Cube game object.
3. Inspector > Mesh Renderer > Lightmapping.
4. Shift + Click on Baked Lightmap tab.

Actual Result:
RGB, R and G tabs are missing.

Expected Result:
RGB, R and G tabs are present.

Reproducible in:
2020.1.0a4, 2019.3.0b4.

Working Fine in:
2020.1.0a3, 2019.3.0b3.

Environment:
Windows & Mac 10.14.