fix: explicit default for event handler

rust derives default of T: Default, even when that's not a necessary requirement

This fixes that

packages/core/src/nodes.rs

@@ -351,11 +351,18 @@ type ExternalListenerCallback<'bump, T> = BumpBox<'bump, dyn FnMut(T) + 'bump>;
 /// }
 ///
 /// ```
-#[derive(Default)]
 pub struct EventHandler<'bump, T = ()> {
     pub callback: RefCell<Option<ExternalListenerCallback<'bump, T>>>,
 }
 
+impl<'a> Default for EventHandler<'a> {
+    fn default() -> Self {
+        Self {
+            callback: RefCell::new(None),
+        }
+    }
+}
+
 impl<T> EventHandler<'_, T> {
     /// Call this event handler with the appropriate event type
     pub fn call(&self, event: T) {