Fix crash in ViewImageFragment, fix #1876 (#1877)

It was not checked whether captionSheet is present.

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

@@ -172,7 +172,7 @@ class ViewImageFragment : ViewMediaFragment() {
     }
 
     override fun onToolbarVisibilityChange(visible: Boolean) {
-        if (photoView == null || !userVisibleHint) {
+        if (photoView == null || !userVisibleHint || captionSheet == null) {
             return
         }
         isDescriptionVisible = showingDescription && visible
@@ -180,7 +180,7 @@ class ViewImageFragment : ViewMediaFragment() {
         captionSheet.animate().alpha(alpha)
                 .setListener(object : AnimatorListenerAdapter() {
                     override fun onAnimationEnd(animation: Animator) {
-                        captionSheet.visible(isDescriptionVisible)
+                        captionSheet?.visible(isDescriptionVisible)
                         animation.removeListener(this)
                     }
                 })