Cancel pending resolveChanges dialog when a new one is started

We run a nested event loop while the message dialog is shown and is waiting for user input. When during this a new request to open another module happened (via dbus) we would show a second dialog and run another event loop. After the first dialog was answered with discard changes, the module was destroyed, so the second discard answer chrashes trying to reset module which was no longer around. Instead prevent this by rejecting the first dialog (effect being staying on the current module and doing nothing) before showing the new one. This also ensures that afterwards the correct module is opened (the one from the second request). BUG:465510 FIXED-IN:5.27.1