Game crashes when cycling spells in spellbook

[AlexJ314]

ArsMagica2/src/main/java/am2/spell/SpellUtils.java

Lines 487 to 488 in 6d6b680

	int max = stack.stackTagCompound.getInteger(NumShapeGroups_Identifier);
	if (max == 0)

Looking at the crash log, it turns out that the variable "max" can hold the value of null, crashing Minecraft with a null pointer exception.
To fix this crash simply, check to make sure the variable "max" isn't null before comparing to 0.

Edit: I'm playing 1.7.10