Steps to reproduce:

1. Open Unity on OSX;
2. Create new project and scene;
3. Add Terrain GameObject -> 3D Object -> Terrain;
4. Select Terrain GameObject in the Hierarchy view;
5. Go to Terrain Settings tab in the Inspector;
6. Click 'Import Raw' button;
7. Close 'Import Raw' window;
8. Observe the console.

Actual result:

Errors:
- InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Peek () (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections/Stack.cs:321)

and

- InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Pop () (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections/Stack.cs:329)

are displayed in the console.

Also this bug reproduces when selecting and opening raw terrain heightmap (from 'Import RAW' window).

See attached gif video and actual.png screenshot.

Regression introduced somewhere in 5.4.0.

Expected result:
Errors should not be thrown when closing or opening raw terrain heightmaps.

Notes:
- Reproduces on 5.4.0b1 and 5.4.0a6;
- Doesn't reproduce on 5.3.0f4 and 5.3.1p1;
- Reproduces only on OSX (10.11.2 checked). MacBook Pro with Retina display with Intel Iris Pro 1536 MB GPU;
- This bug seems not to affect Import Raw heightmap terrain functionality.