Edit show dialog hangs if some resources in the description can't be loaded

[equinox0815]

When a show description uses external resources (i.e. images) which are not accessible at the time of opening the edit show dialog the dialog hangs and doesn't allow to change some settings because the select fields don't get populated properly.

Steps to reproduce:

• Edit a show and add an external resource to the long description which is not reachable (most probably the server hosting the resource must not be available at all because if it is it would fastly respond with a 404?)
• click 'save'
• reopen the dialog: selection of owner, hosts, etc doesn't work anymore

The only workaround is to edit the long description - removing the faulty resource and saving the show. After that the edit show dialog can be reopened and the desired changes can be made (at this point the previously removed resource can be re-added as well)