WebClient does not support concurrent I/O operations

[jng]

Steps to reproduce:

1. Bring up the properties of a resource
2. Switch to the references tab.
3. Cancel the dialog before the references are loaded.
4. Try again. The error occurs. Doing anything afterwards will repeatedly throw this error.

Now the actual error itself may actually be legitimate, but there should be some mechanism whether in Maestro or MaestroAPI itself such that it never enters such an invalid state. (a hanging client connection)

[ksgeograf]

Fixed in r4341.

Maestro now protects against this in the mentioned situation. The exception would be thrown until the operation has completed (which may take a long time).

I will create a new issue to request that MaestroAPI becomes threadsafe.