android: only navigate to main if navController is initialized

Updates #cleanup

Signed-off-by: Percy Wegmann <percy@tailscale.com>
(cherry picked from commit 698fb868a7)

android/src/main/java/com/tailscale/ipn/MainActivity.kt

@@ -265,9 +265,11 @@ class MainActivity : ComponentActivity() {
   override fun onNewIntent(intent: Intent?) {
     super.onNewIntent(intent)
     if (intent?.getBooleanExtra(START_AT_ROOT, false) == true) {
+      if (this::navController.isInitialized) {
         navController.popBackStack(route = "main", inclusive = false)
       }
     }
+  }
 
   private fun login(urlString: String) {
     // Launch coroutine to listen for state changes. When the user completes login, relaunch