[BUG] Fix issue when empty main menu and right click into it

[Hofknecht]

Normally it is not possible to have a empty main menu, you get a message that folder is empty.
But it is possible via adding folder content in options.

Then right click in the menu raises this exception:
System.DivideByZeroException: Attempted to divide by zero. at System.Decimal.DecCalc.VarDecDiv(DecCalc...

consider to change that empty folder is not allowed.
we made this here #192 to directly let the user set up a new folder.
Now i think maybe in the future the menu could be empty and you can fill the empty folder directly in STM