1) What happened
Initializing static membervariables at the declaration causes Flash Conversion error. Setting that variable to null elsewhere in code, then immediately checking if that variable is null, will fail in Flash.

2) How can we reproduce it using the example you attached
Small Unity repro project attached.
Run in Unity, get expected results (see log)
Export to Flash and note the failure. (see flashlog.txt, it shows the failurel)