Editor freezes when using multiple Post Process Volumes with Cubemaps assigned

When using a cubemaps assigned to a Post Process Volume and loading the level the editor will freeze if it's been a project converted from 4.11 to 4.12/4/13. In some instances if the camera is outside of the PPV and you move the camera back into it the editor will freeze at that point.

Tested with:
Release 4.12.2 - CL-3005592
Dev-Main - CL-3007689

1. Create a new project
2. In the "View Options" section of the content browser, select "Show Engine Content".
3. Add two Post Process Volumes to the level
4. Set the first post process volume to Unbound, set an Ambient Cubemap and set the Intensity setting under Ambient Cubemap to true
5. Set the second post process volume's Ambient Cubemap to any Cubemap that is different than the one selected for the previous volume and set its Intensity to true as well
6. Press Play and enter the second Post Process Volume
   Result: The editor freezes and must be closed with the Task Manager
   Expected: The transition between the cubemaps should work without a hitch

Freq: 3/3

Regression: Yes

Overlapping Ambient Cubemap Post Process Volume Crash

Get error when use 2 post process in the same level

Enabling viewport freezes engine