Avoid some NRE's in the logs.

This is more to keep our debug logs a little cleaner.

The worst one was processPluginInterop, it caused an NRE flood upon disconnect.
CheckEditorLock caused exceptions when moving the mouse over the KMP windows before the editor initialized (which takes a second or two) - but this change is mainly to reduce the log noise.

If we are testing for disconnections we should be testing tcpClient for null, as disconnect calls TcpClient.Close() which happens to disposes the object, which causes an NRE upon if-testing anywhere else in the code.