BadWindow XError at startup with 4.16.1

Suddenly encountered xfwm sigtrap after logging out and logging in or reboot.

It seems probabilistic, but I've seen this three times during last week.

(xfwm4:15770): Gdk-ERROR **: 10:42:06.244: The program 'xfwm4' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadWindow (invalid Window parameter)'.
  (Details: serial 2675 error_code 3 request_code 3 (core protocol) minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the GDK_SYNCHRONIZE environment
   variable to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)

From the XErrorDB, the request_code 3 should be XRequest.3:X_GetWindowAttributes.

Attach the patch here for review since I don't have a contribute account.

Hope it can fix the issue...

0001-client-trap-XError-related-to-XGetWindowAttributes.patch