fix merge (#143)

Author: zyf051520

src/main/java/com/cleanroommc/modularui/widget/Widget.java

@@ -311,9 +311,10 @@ public W onUpdateListener(Consumer<W> listener) {
 
     public W onUpdateListener(Consumer<W> listener, boolean merge) {
         if (merge && this.onUpdateListener != null) {
+            final Consumer<W> oldListener = this.onUpdateListener;
             if (listener != null) {
                 this.onUpdateListener = w -> {
-                    this.onUpdateListener.accept(w);
+                    oldListener.accept(w);
                     listener.accept(w);
                 };
             }