Fixes #18782: properly check if htmx_url is None

If this is done incorrently, then the string formatting operation turns
`htmx_url` into a string and the test in the template fails.

netbox/extras/dashboard/widgets.py

@@ -257,7 +257,7 @@ class ObjectListWidget(DashboardWidget):
             parameters['per_page'] = page_size
         parameters['embedded'] = True
 
-        if parameters:
+        if parameters and htmx_url is not None:
             try:
                 htmx_url = f'{htmx_url}?{urlencode(parameters, doseq=True)}'
             except ValueError:

netbox/extras/tests/test_dashboard.py

@@ -0,0 +1,43 @@
+from django.test import tag, TestCase
+
+from extras.dashboard.widgets import ObjectListWidget
+
+
+class ObjectListWidgetTests(TestCase):
+    @tag('regression')
+    def test_widget_fails_gracefully(self):
+        """
+        Example:
+        '2829fd9b-5dee-4c9a-81f2-5bd84c350a27': {
+            'class': 'extras.ObjectListWidget',
+            'color': 'indigo',
+            'title': 'Object List',
+            'config': {
+                'model': 'extras.notification',
+                'page_size': None,
+                'url_params': None
+            }
+        }
+        """
+        config = {
+            # 'class': 'extras.ObjectListWidget',  # normally popped off, left for clarity
+            'color': 'yellow',
+            'title': 'this should fail',
+            'config': {
+                'model': 'extras.notification',
+                'page_size': None,
+                'url_params': None,
+            },
+        }
+
+        class Request:
+            class User:
+                def has_perm(self, *args, **kwargs):
+                    return True
+
+            user = User()
+
+        mock_request = Request()
+        widget = ObjectListWidget(id='2829fd9b-5dee-4c9a-81f2-5bd84c350a27', **config)
+        rendered = widget.render(mock_request)
+        self.assertTrue('Unable to load content. Invalid view name:' in rendered)