properly handle child switch in SingleChildWidget

Author: brachy84

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

@@ -29,12 +29,15 @@ public W child(IWidget child) {
         if (child == this || this.child == child) {
             return getThis();
         }
-
+        if (this.child != null) {
+            this.child.dispose();
+        }
         this.child = child;
-        if (isValid()) {
+        updateList();
+        if (child != null && isValid()) {
             child.initialise(this, true);
+            scheduleResize();
         }
-        updateList();
         return getThis();
     }
 }