More helpful error message when passing a non-component to the component template tag

wagtail/admin/templatetags/wagtailadmin_tags.py

@@ -739,7 +739,10 @@ def component(context, obj, fallback_render_method=False):
     # called instead (with no arguments) - this is to provide deprecation path for things that have
     # been newly upgraded to use the component pattern.
 
-    if fallback_render_method and not hasattr(obj, 'render_html') and hasattr(obj, 'render'):
+    has_render_html_method = hasattr(obj, 'render_html')
+    if fallback_render_method and not has_render_html_method and hasattr(obj, 'render'):
         return obj.render()
+    elif not has_render_html_method:
+        raise ValueError("Cannot render %r as a component" % (obj,))
 
     return obj.render_html(context)

wagtail/admin/tests/test_templatetags.py

@@ -149,3 +149,12 @@ class TestComponentTag(TestCase):
         )
         html = template.render(Context({'my_component': MyComponent()}))
         self.assertEqual(html, "<h1>Look, I&#x27;m running with scissors! 8&lt; 8&lt; 8&lt;</h1>")
+
+    def test_error_on_rendering_non_component(self):
+        template = Template(
+            "{% load wagtailadmin_tags %}<h1>{% component my_component %}</h1>"
+        )
+
+        with self.assertRaises(ValueError) as cm:
+            template.render(Context({'my_component': "hello"}))
+        self.assertEqual(str(cm.exception), "Cannot render 'hello' as a component")