Handle non-event on_ fields in diff and protocol logic

packages/flet/lib/src/controls/list_view.dart

@@ -56,10 +56,11 @@ class _ListViewControlState extends State<ListViewControl> {
         widget.control.getBool("build_controls_on_demand", true)!;
     var firstItemPrototype =
         widget.control.getBool("first_item_prototype", false)!;
-    var prototypeItem = firstItemPrototype
-        ? widget.control.buildWidget("prototype_item")
-        : null;
     var controls = widget.control.children("controls");
+    var prototypeItem = widget.control.buildWidget("prototype_item") ??
+        (firstItemPrototype && controls.isNotEmpty
+            ? ControlWidget(control: controls.first)
+            : null);
 
     Widget listView = LayoutBuilder(
       builder: (BuildContext context, BoxConstraints constraints) {

sdk/python/packages/flet/src/flet/controls/core/list_view.py

@@ -54,6 +54,9 @@ class ListView(LayoutControl, ScrollableControl, AdaptiveControl):
     """
     A fixed height or width (when [`horizontal`][(c).] is `True`)
     of an item to optimize rendering.
+
+    Note: This property has effect only when [`build_controls_on_demand`][(c).]
+        is `True` or [`spacing`][(c).] is `0`.
     """
 
     first_item_prototype: bool = False
@@ -63,6 +66,15 @@ class ListView(LayoutControl, ScrollableControl, AdaptiveControl):
     i.e. their `height` or `width` will be the same as the first item.
     """
 
+    prototype_item: Optional[Control] = None
+    """
+    A control to be used as a "prototype" for all items,
+    i.e. their `height` or `width` will be the same as the `prototype_item`.
+
+    Note: This property has effect only when [`build_controls_on_demand`][(c).]
+        is `True` or [`spacing`][(c).] is `0`.
+    """
+
     divider_thickness: Number = 0
     """
     If greater than `0` then `Divider` is used as a spacing between list view items.