Add nullability check to video capture callback.

Fixes #12666

app/src/main/java/org/thoughtcrime/securesms/mediasend/CameraXFragment.java

@@ -2,6 +2,7 @@ package org.thoughtcrime.securesms.mediasend;
 
 import android.animation.Animator;
 import android.annotation.SuppressLint;
+import android.app.Activity;
 import android.content.Context;
 import android.content.pm.ActivityInfo;
 import android.content.res.Configuration;
@@ -448,7 +449,10 @@ public class CameraXFragment extends LoggingFragment implements CameraFragment {
                                                         @NonNull View flipButton,
                                                         @NonNull Animation inAnimation)
   {
-    requireActivity().runOnUiThread(() -> {
+    Activity activity = getActivity();
+
+    if (activity != null) {
+      activity.runOnUiThread(() -> {
         captureButton.setEnabled(true);
         flashButton.startAnimation(inAnimation);
         flashButton.setVisibility(View.VISIBLE);
@@ -456,6 +460,7 @@ public class CameraXFragment extends LoggingFragment implements CameraFragment {
         flipButton.setVisibility(View.VISIBLE);
       });
     }
+  }
 
   private void onCaptureClicked() {
     Stopwatch stopwatch = new Stopwatch("Capture");