I try get list of childrens for window xyz:

XQueryTree(do->xdpy, window, &dummy, &dummy, &children, &nchildren);

Call return many n umbers of windows, ok. Such as: { 1, 2, 3, 4 }

I do query this windows for isviewable or other property, or any other operation:

X Error of failed request:  BadWindow (invalid Window parameter)
  Major opcode of failed request:  42 (X_SetInputFocus)
  Resource id in failed request:  0x5
  Serial number of failed request:  16
  Current serial number in output stream:  16

Yes, this is not an actual message, but the problem is same. 

If try to access an ummaped (or similar state) window, XTest/XLib print an error message and app. exit to system immediately. 

I don't find any method for check "state of an unmapped window". 

If a window is not accesible, why its listed with "XQueryTree"? Or if its listed in this structure, we can access and find its a "unmapped window". This case has a great pitfall for testing apps, i thing this.

A XGetWindowAttributes call to "unmapped window" must be return a error, or return a valid attribute struct. We must look its property "isviewable" or etc. It's real? 

But currently its returned a fatal-error and XLib call "exit(1)" syscall on _XError, _XDefaultError functions. This reason make impossible for generic window test application development..