Select destination realm from a drop down list when creating a task

When creating a (pull) task, if you have to explicitly specify the realm. However, a better idea would be if the user could select the realm from a dropdown list.

Incidentally, when currently creating a pull task, if you fail to specify the realm you get an error message:

org.apache.syncope.common.lib.SyncopeClientException: InvalidPath [MalformedPathException: Malformed path: null]

This should be updated to let the user know the realm needs to be specified. But this is probably not required if the user has to select the realm from a list.