Fix issue with fast delete of field content

When a field content is deleted too quickly (before the sync mechanism
    happens), an exception is raised in the Python side.
This behavior doesn't occur if you type a value in a field, wait a
little bit that the sync happens, and delete it.
What's happening is that the code related to the netpyne field tries to
delete a key that doesn't exists yet (see the video below).

The fix removes this behavior (see video below)

Author: aranega

webapp/components/general/NetPyNEField.js

@@ -46,7 +46,11 @@ export default class NetPyNEField extends Component {
         !this.model.split('.')[0].startsWith('simConfig')
         || this.model.split('.')[1].startsWith('analysis')
       ) {
-        Utils.execPythonMessage(`del netpyne_geppetto.${this.model}`);
+        Utils.execPythonMessage(`
+        try:
+          del netpyne_geppetto.${this.model}
+        except KeyError:
+          ...`);
       }
     }
     return value;