Crash during call if no STUN server is available

Steps to reproduce

1. Remove STUN server from Talk configurations on the Nextcloud instance. Don't use the trash symbol! Only remove the string from the text field.
2. Freshly start NC Talk for Android
3. Start a call

Expected behaviour

App informs that no call can be made because no STUN server is configured. The administrator must be contacted

Actual behaviour

App crashes

Device brand and model

Samsung SM-A326B

Android version

13

Nextcloud Talk app version

15.0.0 RC3

Nextcloud server version

24.0.5

Talk version

14.0.5

Custom Signaling server configured

No response

Custom TURN server configured

No

Custom STUN server configured

No

Android logs

No response

Server log

No response

Additional information

The stacktrace:

java.lang.NullPointerException: Attempt to invoke virtual method 'org.webrtc.PeerConnection$IceConnectionState org.webrtc.PeerConnection.iceConnectionState()' on a null object reference
	at com.nextcloud.talk.activities.CallActivity.setupVideoStreamForLayout(CallActivity.java:2349)
	at com.nextcloud.talk.activities.CallActivity.lambda$processUsersInRoom$14$com-nextcloud-talk-activities-CallActivity(CallActivity.java:1904)
	at com.nextcloud.talk.activities.CallActivity$$ExternalSyntheticLambda20.run(Unknown Source:4)
	at android.app.Activity.runOnUiThread(Activity.java:7258)
	at com.nextcloud.talk.activities.CallActivity.processUsersInRoom(CallActivity.java:1903)
	at com.nextcloud.talk.activities.CallActivity.receivedSignalingMessage(CallActivity.java:1650)
	at com.nextcloud.talk.activities.CallActivity.receivedSignalingMessages(CallActivity.java:1634)
	at com.nextcloud.talk.activities.CallActivity.access$2300(CallActivity.java:185)
	at com.nextcloud.talk.activities.CallActivity$13$1.onNext(CallActivity.java:1494)
	at com.nextcloud.talk.activities.CallActivity$13$1.onNext(CallActivity.java:1484)
	at io.reactivex.internal.operators.observable.ObservableRetryPredicate$RepeatObserver.onNext(ObservableRetryPredicate.java:69)
	at io.reactivex.internal.operators.observable.ObservableTakeWhile$TakeWhileObserver.onNext(ObservableTakeWhile.java:88)
	at io.reactivex.internal.util.HalfSerializer.onNext(HalfSerializer.java:107)
	at io.reactivex.internal.operators.observable.ObservableRepeatWhen$RepeatWhenObserver.onNext(ObservableRepeatWhen.java:100)
	at io.reactivex.internal.operators.observable.ObservableObserveOn$ObserveOnObserver.drainNormal(ObservableObserveOn.java:201)
	at io.reactivex.internal.operators.observable.ObservableObserveOn$ObserveOnObserver.run(ObservableObserveOn.java:255)
	at io.reactivex.android.schedulers.HandlerScheduler$ScheduledRunnable.run(HandlerScheduler.java:124)
	at android.os.Handler.handleCallback(Handler.java:938)
	at android.os.Handler.dispatchMessage(Handler.java:99)
	at android.os.Looper.loopOnce(Looper.java:226)
	at android.os.Looper.loop(Looper.java:313)
	at android.app.ActivityThread.main(ActivityThread.java:8751)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:571)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1135)

There is also a related issue in spreed: https://github.com/nextcloud/spreed/pull/8020
But also if that one is fixed, it must handled better in the client.

Start with CallActivity.java, especially setupVideoStreamForLayout at line 2349 and the processUsersInRoom path shown in the stack trace. Reproduce a call with the STUN field empty and inspect the peer-connection state access. Done means the app no longer crashes and informs the user that calling cannot proceed because no STUN server is configured.