Instead of destroying the BAboutWindow object on close, Hide() it, then on the destructor of the calling window call Quit() explicitly to destroy it.

src/apps/activitymonitor/ActivityView.cpp

@@ -1112,22 +1112,16 @@ ActivityView::MessageReceived(BMessage* message)
 
 	switch (message->what) {
 		case B_ABOUT_REQUESTED:
-		{
-			bool quit = false;
-			if (message->FindBool("quit", &quit) == B_OK && quit) {
-				fAboutWindow = NULL;
-				break;
-			}
-
 			if (fAboutWindow == NULL) {
-				fAboutWindow = new BAboutWindow(this, kAppName, kSignature);
+				fAboutWindow = new BAboutWindow(kAppName, kSignature);
 				fAboutWindow->AddCopyright(2008, "Haiku, Inc.");
 				fAboutWindow->Show();
-			} else
+			} else if (fAboutWindow->IsHidden())
+				fAboutWindow->Show();
+			else
 				fAboutWindow->Activate();
 
 			break;
-		}
 
 		case kMsgUpdateResolution:
 		{