When "Import Raw Heightmap" is closed or Cancel the Importing Raw Heightmap from "Terrain Settings", then it causes the editor to throw the following errors:
"EndLayoutGroup: BeginLayoutGroup must be called first.
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)".
Refer the attached video

Steps to repro:
1. Create a new project.
2. Hierarchy > Create > 3D Object > Terrain
3. Terrain > Inspector > Terrain Settings
4. Texture Resolution (On Terrain Data) > Import Raw > Click
5. Closed "Import Raw Heightmap" window OR Cancel > Click

Actual Result:
"EndLayoutGroup: BeginLayoutGroup must be called first." error is thrown when "Import Raw Heightmap" window is closed OR Cancel the importing

Expected Result:
"Import Raw Heightmap" window is closed without throwing any error in the console

Occurring on:
2020.2.0a12

Working fine:
2020.2.0a11, 2020.1.0b9

Environment:
Occurs only on Windows 10