Can't enter a seed if, at some point, a sheet can't be generated

[laszlostark]

When entering a seed, a sheet is generated every time a letter is typed into the seed textfield. If at any one of those steps a sheet can't be generated (which happens somewhat frequently on hard still), a popup is displayed and another random seed is automatically chosen.

This is the case, for example, with the seed 63133 on Hard difficulty. Type it in, letter by letter, and you'll notice that 63 fails to generate a sheet, at which point the alert displays and a new seed is chosen.