FocusMap is one of the last things to get populated as a map document is loaded. At what point is your code being called? If your tool is listening to document events you would want to get a reference to the map after IDocumentEvents.OpenDocument and/or IActiveViewEvents.FocusMapChanged have fired. If you don't need the reference to FocusMap until the tool is being used anyway, there's nothing wrong with calling it from your MyToolClass as you have already done.
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