When using a crafting grid in any system, if you place blocks then click & drag over them with the same block it crashes.

This is the exact line, the variable it's accessing that's throwing the variable is this.dragSplittingLimit as that exists in the Minecraft namespace and is private.
My debugger reports the line as: (AEBaseGui.java:970)

Here's the blame because I'm a snitch: https://github.com/AE2-UEL/Applied-Energistics-2/blame/master/src/main/java/appeng/client/gui/AEBaseGui.java#L970

Crash Log: https://mclo.gs/CEjojbp
AE2 Version: v0.56.6 (only AE2 installed)