check nullability of assistant

Signed-off-by: alperozturk96 <alper_ozturk@proton.me>

Author: alperozturk96

app/src/main/java/com/owncloud/android/ui/activity/DrawerActivity.java

@@ -325,7 +325,13 @@ private void themeBottomNavigationMenu() {
 
     @SuppressFBWarnings("RV")
     private void checkAssistantBottomNavigationMenu() {
-        boolean isAssistantAvailable = getCapabilities().getAssistant().isTrue();
+        final var capability = getCapabilities();
+        boolean isAssistantAvailable;
+        if (capability == null) {
+            isAssistantAvailable = false;
+        } else {
+            isAssistantAvailable = capability.getAssistant().isTrue();
+        }
 
         bottomNavigationView
             .getMenu()