fix crash in ViewImageFragment (#4686)

From Play console crash logs. Well, let's put another band aid on this
code....

```
Exception java.lang.IllegalStateException:
  at androidx.fragment.app.Fragment.requireActivity (Fragment.java:1005)
  at com.keylesspalace.tusky.fragment.ViewImageFragment.getPhotoActionsListener (ViewImageFragment.java:59)
  at com.keylesspalace.tusky.fragment.ViewImageFragment.access$getPhotoActionsListener (ViewImageFragment.java:49)
  at com.keylesspalace.tusky.fragment.ViewImageFragment$onViewCreated$singleTapDetector$1.onSingleTapConfirmed (ViewImageFragment.kt:116)
  at android.view.GestureDetector$GestureHandler.handleMessage (GestureDetector.java:379)
  at android.os.Handler.dispatchMessage (Handler.java:106)
  at android.os.Looper.loopOnce (Looper.java:230)
  at android.os.Looper.loop (Looper.java:319)
  at android.app.ActivityThread.main (ActivityThread.java:8919)
  at java.lang.reflect.Method.invoke
  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:578)
  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:1103)
```

app/src/main/java/com/keylesspalace/tusky/fragment/ViewImageFragment.kt

@@ -112,7 +112,9 @@ class ViewImageFragment : ViewMediaFragment() {
             object : GestureDetector.SimpleOnGestureListener() {
                 override fun onDown(e: MotionEvent) = true
                 override fun onSingleTapConfirmed(e: MotionEvent): Boolean {
-                    photoActionsListener.onPhotoTap()
+                    if (isAdded) {
+                        photoActionsListener.onPhotoTap()
+                    }
                     return false
                 }
             }