Fix exception on clicking Play before dropdown appears

[HebaruSan]

Problem

Cause

Main.LaunchGame implicitly assumes its string argument will not be null.

ManageMods.LaunchGameToolStripMenuItem_Click passes menuItem?.Tag as string, which implicitly assumes the menu item's Tag will always be set.

The tag is set in LaunchGameToolStripMenuItem_MouseHover, which is executed when you hover the button, but after a short delay. If you click before the delay is over, the tag is null.

Changes

Now Main.LaunchGame allows a null argument and defaults to the first configured command line.