Using CommandBuffer:IssuePluginEvent(Async) in OnRenderImage makes editor unresponsive on quit

The CommandBuffer::IssuePluginEvent(Async) seems to work fine, but once play is stopped, quitting the editor makes it unresponsive (as well as opening a new project or attaching the unity debugger)

The Visual Studio debugger writes: Your app has entered a break state, but there is no code to show because all threads were executing external code (typically system or framework code), but does on occasion show a stack trace:

[external code]
 mono.dll!mono_thread_suspend_all_other_threads() Line 3412
     at c:\buildslave\mono\build\mono\metadata	hreads.c(3412)
 mono.dll!mono_unity_jit_cleanup(_MonoDomain * domain) Line 1946
     at c:\buildslave\mono\build\mono\mini\driver.c(1946)
 [external code]

This is using Unity 2018.3.0b7 on windows 10 version 1809, D3D11


The problem was caused by the c++ function called through the CommandBuffer::IssuePluginEvent(Async) calling back into a C# function to print stuff to the Debug.Log.
Not doing that made things work.