Handle absent notification managers and nullable call channels explicitly

tools/chatgui-android/app/src/main/java/com/utun/chat/data/CallService.kt

@@ -66,7 +66,7 @@ class CallService : Service() {
     }
 
     private fun createChannel() {
-        val manager = getSystemService(NotificationManager::class.java)
+        val manager = requireNotNull(getSystemService(NotificationManager::class.java)) { "notification manager unavailable" }
         manager.createNotificationChannel(
             NotificationChannel(CHANNEL, AppLanguage.text(R.string.ongoing_call), NotificationManager.IMPORTANCE_DEFAULT).apply {
                 description = AppLanguage.text(R.string.ongoing_call_description)
@@ -77,7 +77,7 @@ class CallService : Service() {
         val channel = manager.getNotificationChannel(CHANNEL)
         LogManager.addLog("INFO", "CallService",
             "notification channel=$CHANNEL enabled=${manager.areNotificationsEnabled()} " +
-                "importance=${channel.importance} lockscreenVisibility=${channel.lockscreenVisibility}")
+                "importance=${channel?.importance} lockscreenVisibility=${channel?.lockscreenVisibility}")
     }
 
     fun refreshLanguage() { createChannel(); updateNotification() }
@@ -164,7 +164,8 @@ class CallService : Service() {
     }
 
     fun updateNotification() {
-        getSystemService(NotificationManager::class.java).notify(NOTIFICATION_ID, notification())
+        requireNotNull(getSystemService(NotificationManager::class.java)) { "notification manager unavailable" }
+            .notify(NOTIFICATION_ID, notification())
         LogManager.addLog("INFO", "CallService", "notification updated id=$callId phase=${CallController.state.value?.phase}")
     }