Fix pointer capture code, and add check to catch errors earlier. (#1074)

Author: PoignardAzur

masonry_core/src/passes/event.rs

@@ -22,7 +22,9 @@ fn get_pointer_target(
 ) -> Option<WidgetId> {
     // See the [pointer capture documentation](../doc/06_masonry_concepts.md#pointer-capture).
     if let Some(capture_target) = root.global_state.pointer_capture_target {
-        return Some(capture_target);
+        if root.widget_arena.has(capture_target) {
+            return Some(capture_target);
+        }
     }
 
     if let Some(pointer_pos) = pointer_pos {