android/ui: prevent navigation before we've added content

Fixes tailscale/corp#19070

If the activity hadn't yet been created, we can still get an onIntent which
was assuming the navController had been instantiated.  Switched that to
and optional so that we can null check it.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

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

@@ -74,11 +74,10 @@ import kotlinx.coroutines.launch
 
 class MainActivity : ComponentActivity() {
   private lateinit var requestVpnPermission: ActivityResultLauncher<Unit>
-  private lateinit var navController: NavHostController
+  private var navController: NavHostController? = null
 
   companion object {
     private const val TAG = "Main Activity"
-
     private const val START_AT_ROOT = "startAtRoot"
   }
 
@@ -245,7 +244,7 @@ class MainActivity : ComponentActivity() {
   override fun onNewIntent(intent: Intent?) {
     super.onNewIntent(intent)
     if (intent?.getBooleanExtra(START_AT_ROOT, false) == true) {
-      navController.popBackStack(route = "main", inclusive = false)
+      navController?.popBackStack(route = "main", inclusive = false)
     }
   }