Cancel loading unsupported ROM, get Error

What steps will reproduce the problem?
1. Open a ROM that's not supported.  I chose Jaguar.  This can also work for 
ROMs not in the GameDB.
2. When asked, "What core is this for?" select Cancel.
3. Error

What is the expected output? What do you see instead?
EmuHawk to go back to it's previous state (null emulator or current Core)

Error message
A core accepted the rom, but threw an exception while loading it:

System.ArgumentNullException: Value cannot be null.
Parameter name: key
   at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at BizHawk.Emulation.Cores.CoreInventory.get_Item(String system)
   at BizHawk.Client.Common.RomLoader.LoadRom(String path, CoreComm nextComm, Boolean forceAccurateCore)

What version of the product are you using? On what operating system?
r8215

Please provide any additional information below.

Original issue reported on code.google.com by hegyak on 20 Nov 2014 at 11:50

divyang4481 / bizhawk

Cancel loading unsupported ROM, get Error #316