Forms - Dialog Box Shows When Function Returns Nothing And Sub Doesn't Exit

I am having trouble with some code I am writing to handle an Excel spreadsheet. The idea is that the dialog box is displayed which invites the user to select an Excel spreadsheet. Once they have done that, it is opened and a separate function checks to see whether Sheet1 exists returning Nothing if it is not found. The intention being the try/catch block picks it up and gracefully does nothing. The problem is that when the function returns nothing, the dialog box is still displayed and the sub doesn't seem to exit. [Code]