Actually, this should be enough

src/main/kotlin/ru/dbotthepony/mc/otm/compat/ExtendedInventoryHandler.kt

@@ -186,6 +186,18 @@ private class FakeSlot : MatterySlot(SimpleContainer(1), 0, 0, 0) {
 	override fun allowModification(p_150652_: Player): Boolean {
 		return false
 	}
+
+	override fun isActive(): Boolean {
+		return false
+	}
+
+	override fun getMaxStackSize(pStack: ItemStack): Int {
+		return 0
+	}
+
+	override fun getMaxStackSize(): Int {
+		return 0
+	}
 }
 
 class InventoryScrollPacket(val scroll: Int) : MatteryPacket {