Fix MAGN-4095 Getting crash while running Document after closing and reopening again.

@lukechurch

This submission fixes the crash when Dynamo tries to read trace data using SoapFormatter. The formatter will require the type information related to the object to be desialized and it will try to load the assembly. But unfortunately if we close the plugin, the resolver is removed. As a result, the assembly will not be found and the formatter will throw an exception. Ideally the resolver should be removed when the plugin is shutdown. This submission makes the fix the remove the resolver at the right time.
@ikeough 

There are two concepts: plugin and command. The first time you start the plugin, the plugin will be initiated and only when the plugin is unloaded, the shutdown will be called. Each of this will only be called once. But in one session, users may close Dynamo or re-open Dynamo. The command will be run multiple times or the view closing event will be triggered multiple times.

Since the registration of the resolver is done when the plugin is initiated, it will only be registrated once even if we close and reopen Dynamo multiple times.