In the intro, wait for the derivation to be complete before continuing

app/src/main/java/me/impy/aegis/IntroActivity.java

@@ -109,6 +109,10 @@ public class IntroActivity extends AppIntro implements DerivationTask.Callback {
     @Override
     public void onDonePressed(Fragment currentFragment) {
         super.onDonePressed(currentFragment);
+        // wait for the key derivation background task
+        if (_passwordSlot == null || _passwordCipher == null) {
+            return;
+        }
 
         int cryptType = _authenticatedSlide.getCryptType();