Fix crash caused by using Settings->Configure Toolbars

This code assumed that the Action::FileNew button would be present, but when the button was removed a null-pointer dereference occurred. This fix is incomplete, because there appear to be additional unsafe references to this (and possibly other) buttons.

I've recently started using KMyMoney, and I have a sizable list of bugs some of which I'd like to try to just fix. This particular bug required some debugging just so I could progress to doing development on other bugs. ;-) My apologies for the incomplete nature of this fix, but I'm not familiar enough with the code to do a review of the other places affected. I know that switching to the Institutions view also caused a crash because of the customized toolbar. I was able to work around this defect by restoring the toolbar configuration to the default setting once KMyMoney would start successfully.