How to reproduce this:
1) Install ReactOS as normally way
2) At 3rd stage wait the new hardware wizard appears (to install System Device) select Install driver from specific location
3) Click Next
4) Select "Include this path"
5) at this point, break debugger y set first chance exception
6) Click on browse button and try to expand My Computer
7) Wait a few seconds and then see your debug log:
Entered debugger on first-chance exception (Exception Code: 0xc0000005) (Page Fault)
Memory at 0x00000000 could not be written: Page not present.
Now you can do a backtrace, meanwhile, ReactOS explorer shows a busy mouse icon.
After finishing the backtrace, and cont the system, the log follows with this:
err:(win32ss/user/user32/windows/message.c:1506) Exception when calling unicode WndProc 7ADFE7DC Msg 78 pti FFB0D9B8 Wndpti FFB0D9B8
Press ALT+TAB and explorer gets usable again.