DBManager topoviewer: don't be fooled by srid=-1 in topology.topology

Fixes #12802

python/plugins/db_manager/db_plugins/postgis/plugins/qgis_topoview/__init__.py

@@ -81,7 +81,11 @@ def run(item, action, mainwindow):
             mainwindow.infoBar.pushMessage("Invalid topology", u'Schema "{0}" is not registered in topology.topology.'.format(item.schema().name), QgsMessageBar.WARNING, mainwindow.iface.messageTimeout())
             return False
 
-        toposrid = str(res[0])
+        if ( res[0] < 0 ):
+          mainwindow.infoBar.pushMessage("WARNING", u'Topology "{0}" is registered as having a srid of {1} in topology.topology, we will assume 0 (for unknown)'.format(item.schema().name, res[0]), QgsMessageBar.WARNING, mainwindow.iface.messageTimeout())
+          toposrid = '0'
+        else:
+          toposrid = str(res[0])
 
         # load layers into the current project
         toponame = item.schema().name