Passing property of object which is null as parameter in command would not start the command but return 200 anyway.

[MichalTichy]

Example:
Foo = null;
<dot:button Click="{command: Test(Foo.Bar}" />
This commands goes will not be run at the server but its request will return 200 regardless.
User is left without any hint why this happened.
We should probably throw exception in such scenarios.