Project crashes upon upgrade from 5.3 to 5.4. Issue seems to be IMGUI related.

Repro steps:

1. Download Turn Based Ball Fight by Finalboss from asset store or open attached project in 5.3
2. Play "Game" scene to make sure it runs.
3. Open project in 5.4.0b7 (spin 3)
4. Unity crashes

Expected outcome: project to update and open.

Notes:
- Regression; project opens up to 5.4.0b4, crashes in b5 onwards.

========== OUTPUTING STACK TRACE ==================

0x000000014120B11F (Unity) TextRenderingPrivate::NativeTextGenerator::InsertCharacter
0x000000014120B6FB (Unity) TextRenderingPrivate::NativeTextGenerator::ProcessString
0x000000014121005A (Unity) TextRenderingPrivate::TextMeshGenerator::Generate
0x00000001412104FE (Unity) TextRenderingPrivate::TextMeshGenerator::Get
0x000000014120CC30 (Unity) TextRenderingPrivate::TextMesh::ApplyToMesh
0x000000014120DF80 (Unity) TextRenderingPrivate::TextMesh::AwakeFromLoad
0x00000001405124A3 (Unity) AwakeFromLoadQueue::InvokePersistentManagerAwake
0x0000000140512B45 (Unity) AwakeFromLoadQueue::PersistentManagerAwakeFromLoad
0x00000001404AB6E1 (Unity) LoadSceneOperation::CompleteAwakeSequence
0x00000001404AB77A (Unity) LoadSceneOperation::CompletePreloadManagerLoadSceneEditor
0x00000001404ACBAA (Unity) LoadSceneOperation::IntegrateMainThread
0x00000001404AB19E (Unity) PreloadManager::UpdatePreloadingSingleStep
0x00000001404AB286 (Unity) PreloadManager::WaitForAllAsyncOperationsToComplete
0x0000000140B4A4A0 (Unity) EditorSceneManager::RestoreSceneBackups
0x0000000140B4ECEE (Unity) EditorSceneManager::RestoreSceneManagerSetup
0x0000000140B4F07E (Unity) EditorSceneManager::LoadSceneManagerSetup
0x0000000140BB5D31 (Unity) Application::FinishLoadingProject
0x0000000140DF7C9B (Unity) WinMain
0x00000001415B3DE4 (Unity) read
0x00007FFDC84A8102 (KERNEL32) BaseThreadInitThunk

========== END OF STACKTRACE ===========