avoid NPE in ChatController#determinePreviousMessageIds

W/System.err: java.lang.NullPointerException
W/System.err:     at com.nextcloud.talk.controllers.ChatController.determinePreviousMessageIds(ChatController.kt:2498)
W/System.err:     at com.nextcloud.talk.controllers.ChatController.processMessagesNotFromTheFuture(ChatController.kt:2463)
W/System.err:     at com.nextcloud.talk.controllers.ChatController.processMessages(ChatController.kt:2355)
W/System.err:     at com.nextcloud.talk.controllers.ChatController.processMessagesResponse(ChatController.kt:2314)
W/System.err:     at com.nextcloud.talk.controllers.ChatController.access$processMessagesResponse(ChatController.kt:219)
W/System.err:     at com.nextcloud.talk.controllers.ChatController$pullChatMessages$3.onNext(ChatController.kt:2260)
W/System.err:     at com.nextcloud.talk.controllers.ChatController$pullChatMessages$3.onNext(ChatController.kt:2248)
W/System.err:     at io.reactivex.internal.operators.observable.ObservableObserveOn$ObserveOnObserver.drainNormal(ObservableObserveOn.java:201)
W/System.err:     at io.reactivex.internal.operators.observable.ObservableObserveOn$ObserveOnObserver.run(ObservableObserveOn.java:255)
W/System.err:     at io.reactivex.android.schedulers.HandlerScheduler$ScheduledRunnable.run(HandlerScheduler.java:124)
W/System.err:     at android.os.Handler.handleCallback(Handler.java:883)
W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:100)
W/System.err:     at android.os.Looper.loop(Looper.java:224)
W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:7590)
W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
W/System.err:     at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)

Signed-off-by: Marcel Hibbe <dev@mhibbe.de>

app/src/main/java/com/nextcloud/talk/controllers/ChatController.kt

@@ -2494,11 +2494,15 @@ class ChatController(args: Bundle) :
 
             if (previousMessageId > NO_PREVIOUS_MESSAGE_ID) {
                 chatMessage.previousMessageId = previousMessageId
-            } else if (adapter?.isEmpty != true) {
-                if (adapter!!.items[0].item is ChatMessage) {
-                    chatMessage.previousMessageId = (adapter!!.items[0].item as ChatMessage).jsonMessageId
-                } else if (adapter!!.items.size > 1 && adapter!!.items[1].item is ChatMessage) {
-                    chatMessage.previousMessageId = (adapter!!.items[1].item as ChatMessage).jsonMessageId
+            } else {
+                adapter?.let {
+                    if (!it.isEmpty) {
+                        if (it.items[0].item is ChatMessage) {
+                            chatMessage.previousMessageId = (it.items[0].item as ChatMessage).jsonMessageId
+                        } else if (it.items.size > 1 && it.items[1].item is ChatMessage) {
+                            chatMessage.previousMessageId = (it.items[1].item as ChatMessage).jsonMessageId
+                        }
+                    }
                 }
             }