Stop repeater identity key search when user navigates away

Terminate a search for a repeater key with a specified prefix if the
user navigates away from the settings screen.

This commit also adds dispose() calls for the new repeater key input
controllers, which were previously omitted.

lib/screens/repeater_settings_screen.dart

@@ -216,6 +216,10 @@ class _RepeaterSettingsScreenState extends State<RepeaterSettingsScreen> {
     _txDelayController.dispose();
     _directTxDelayController.dispose();
     _intThreshController.dispose();
+    _prvKeyController.dispose();
+    _pubKeyController.dispose();
+    _prvKeyPrefixController.dispose();
+    _stopSearchingForKeyPair = true;
     super.dispose();
   }
 
@@ -1032,7 +1036,7 @@ class _RepeaterSettingsScreenState extends State<RepeaterSettingsScreen> {
 
   void _searchForKeyPair() async {
     if (_stopSearchingForKeyPair) {
-      setState(() => _searchingForKeyPair = false);
+      if (mounted) setState(() => _searchingForKeyPair = false);
       return;
     }
     final String prefix = _prvKeyPrefixController.text.trim();