[fix] Fixed "receive_url" field in TopologyAdmin

Bug:
When WiFi Mesh integration is enabled, opening the topology add
view was generating Server 500 Error. The issue was caused because
the topology instance for the ModelForm didn't have primary key set.

Fix:
Do not show "receive_url" field on add view.

openwisp_network_topology/admin.py

@@ -104,6 +104,14 @@ class TopologyAdmin(
     receive_url_baseurl = app_settings.TOPOLOGY_API_BASEURL
     change_form_template = 'admin/topology/topology/change_form.html'
 
+    def get_fields(self, request, obj=None):
+        fields = list(super().get_fields(request, obj))
+        if not obj:
+            # Receive URL cannot be created without an object.
+            # Hence, remove the "receive_url" field.
+            fields.remove('receive_url')
+        return fields
+
     def get_actions(self, request):
         """
         move delete action to last position

openwisp_network_topology/integrations/device/tests/test_wifi_mesh.py

@@ -266,8 +266,14 @@ def test_topology_admin(self):
         """
         admin = self._create_admin()
         self.client.force_login(admin)
-        topology = self._create_topology()
-        response = self.client.get(
-            reverse(f'{self.prefix}_topology_change', args=[topology.id])
-        )
-        self.assertContains(response, 'Wifi mesh')
+
+        with self.subTest('Test add form'):
+            response = self.client.get(reverse(f'{self.prefix}_topology_add'))
+            self.assertContains(response, 'Wifi mesh')
+
+        with self.subTest('Test change form'):
+            topology = self._create_topology()
+            response = self.client.get(
+                reverse(f'{self.prefix}_topology_change', args=[topology.id])
+            )
+            self.assertContains(response, 'Wifi mesh')