fix crash when recycler view is null

mastodon/src/main/java/org/joinmastodon/android/fragments/IsOnTop.java

@@ -0,0 +1,13 @@
+package org.joinmastodon.android.fragments;
+
+import androidx.annotation.Nullable;
+import androidx.recyclerview.widget.RecyclerView;
+
+public interface IsOnTop {
+    boolean isOnTop();
+
+    default boolean isRecyclerViewOnTop(@Nullable RecyclerView list) {
+        if (list == null) return true;
+        return !list.canScrollVertically(-1);
+    }
+}