ui: only render problem selection element when exists

packages/ui-default/pages/problem_main.page.tsx

@@ -454,11 +454,14 @@ const page = new NamedPage(['problem_main'], () => {
   $(document).on('vjContentNew', (e) => processElement(e.target));
   processElement(document);
 
-  ReactDOM.createRoot(document.getElementById('problem_selection'))
+  const selection = document.getElementById('problem_selection');
+  if (selection) {
+    ReactDOM.createRoot(selection)
       .render(<ProblemSelectionDisplay
         onChange={(pids) => { selectedPids = pids; }}
         onClear={(handler) => { clearSelectionHandler = handler; }}
       />);
+  }
 
   addSpeculationRules({
     prerender: [{