cCustomComboBoxFleetGroupTree throws an unhandled exception when Dispose is called

[joeb]

• Open the Run Ecosim form
• Click on the Fishing Effort dropdown combo box.
• Close the application

cCustomComboBoxFleetGroupTree.Clear() throws an exception when called from Dispose. I've put a Try Catch in Clear() and Asserted on the Catch to make it easier to find when this happens. Its most likely not a big deal as this only happens during the call from Dispose().