main.js: Handle an end-session-dialog failure to show.

If the end-session dialog is requested while the screensaver is
active, pushModal() will fail, as cinnamon-screensaver has a hard
keyboard and pointer grab.

Discard the dialog when this happens, to allow a successful prompt
later.

Fixes linuxmint/mint22.1-beta#108.

js/ui/main.js

@@ -1608,7 +1608,10 @@ function showEndSessionDialog(mode) {
     }
 
     endSessionDialog = new EndSessionDialog(mode);
-    endSessionDialog.open();
+    if (!endSessionDialog.open()) {
+        endSessionDialog = null;
+        global.logWarning("Failed to open end session dialog");
+    }
 }
 
 function closeEndSessionDialog() {