diff has 3 places of same (almost) text:

-    Result := nil;
+  if (MonitorHandle = 0) and (MonitorDefault = mdNull) then
+    Result := nil
+  else
+  begin
+    // the user expects some monitor but handle wasn't found -> the
monitor list has probably changed
+    UpdateMonitors;
+    for i := 0 to MonitorCount - 1 do
+      if Monitors[i].Handle = MonitorHandle then
+        Exit(Monitors[i]);
+    RaiseGDBException('TScreen.MonitorFromWindow'); // internal error
+  end;

This is good to put to procedure with AErrorMsg: string.