Steps to reproduce the issue:
1. Download and open attached project.
2. Switch project target to iOS.
3. Open the project in MonoDevelop and set a breakpoint in "AppProxy.cs" line 50 (Start() method).
4. Attach the debugger to Unity.
5. Back in the Editor and open the scene "Watchtower_V3".
6. In the menu navigate to "WBSF -> Game -> Run".
7. Make sure the breakpoint hits and press "Continue". Verify that the Debugger is still attached, sometimes it detaches. If dettached, reattach.
8. Wait for the "Error" Dialog.
9. After seeing the error, press restart a few times.
10. Stop the player (debugger should stay attached).
11. In the menu navigate to "WBSF -> Game -> Run".
12. After the breakpoint was hit again, press "Continue".

Actual result:
Unity freezes.

Expected result:
Unity should not freeze.

Note:
If the issue is not reproducible, repeat the last three steps (10-12) several times.

Reproduced with:
5.3.5p3, 5.3.5p8, 5.3.6f1.

Cannot reproduce with:
5.1.5f1, 5.2.5f1 (because The type or namespace name `SceneManagement' does not exist in the namespace).
5.4.0f3 (because UnityEditor.BuildPipeline.BuildAssetBundles() is obsolete).
5.5.0a5 (after launching the project and agreeing to update the Scripting API the Editor freezes immediately).