Resolve error when specifying 'slug' as a read-only field (#11447)

wagtail/admin/panels/title_field_panel.py

@@ -78,7 +78,11 @@ class TitleFieldPanel(FieldPanel):
                 actions = [widget.attrs.get("data-action", None)] + self.apply_actions
                 attrs["data-action"] = " ".join(filter(None, actions))
 
-            targets = [self.get_target_selector(target) for target in panel.targets]
+            targets = [
+                self.get_target_selector(target)
+                for target in panel.targets
+                if target in self.form.fields
+            ]
             attrs["data-w-sync-target-value"] = ", ".join(filter(None, targets))
 
             placeholder = self.get_placeholder()

wagtail/admin/tests/test_edit_handlers.py

@@ -2260,6 +2260,25 @@ class TestTitleFieldPanel(WagtailTestUtils, TestCase):
             "focus->w-sync#check blur->w-sync#apply change->w-sync#apply keyup->w-sync#apply",
         )
 
+    def test_form_without_slugfield(self):
+        html = self.get_edit_handler_html(ObjectList([TitleFieldPanel("title")]))
+
+        self.assertIsNotNone(html.find(attrs={"class": "w-panel title"}))
+
+        attrs = html.find("input").attrs
+        self.assertEqual(attrs["data-w-sync-target-value"], "")
+
+    def test_form_with_readonly_slugfield(self):
+        html = self.get_edit_handler_html(
+            ObjectList([TitleFieldPanel("title"), FieldPanel("slug", read_only=True)]),
+            instance=EventPage(),
+        )
+
+        self.assertIsNotNone(html.find(attrs={"class": "w-panel title"}))
+
+        attrs = html.find("input").attrs
+        self.assertEqual(attrs["data-w-sync-target-value"], "")
+
     def test_not_using_apply_actions_if_live(self):
         """
         If the Page (or any model) has `live = True`, do not apply the actions by default.