Check the active event loop in wxGenericProgressDialog dtor

Verify that the active loop didn't change during this object lifetime as
otherwise we could deactivate a different event loop from the one we
installed.

It's still a programming error to write code which doesn't destroy
wxGenericProgressDialog early enough, but at least now "just" assert and
leak memory in this case instead of resetting the active event loop and
hanging the program.

Closes #17983.

src/generic/progdlgg.cpp

@@ -697,6 +697,21 @@ wxGenericProgressDialog::~wxGenericProgressDialog()
 
     if ( m_tempEventLoop )
     {
+        // If another event loop has been installed as active during the life
+        // time of this object, we shouldn't deactivate it, but we also can't
+        // delete our m_tempEventLoop in this case because it risks leaving the
+        // new event loop with a dangling pointer, which it will set back as
+        // the active loop when it exits, resulting in a crash. So we have no
+        // choice but to just leak this pointer then, which is, of course, bad
+        // and usually easily avoidable by just destroying the progress dialog
+        // sooner, so warn the programmer about it.
+        wxCHECK_RET
+        (
+            wxEventLoopBase::GetActive() == m_tempEventLoop,
+            "current event loop must not be changed during "
+            "wxGenericProgressDialog lifetime"
+        );
+
         wxEventLoopBase::SetActive(NULL);
         delete m_tempEventLoop;
     }