fix Mastery node right-click crash (#8728)

Fixes #8697 
Temporary fix while runegraft support is finalized.

Author: Nightblade

src/Classes/TreeTab.lua

@@ -798,7 +798,7 @@ function TreeTabClass:ModifyNodePopup(selectedNode)
 	local function buildMods(selectedNode)
 		wipeTable(modGroups)
 		local numLinkedNodes = selectedNode.linkedId and #selectedNode.linkedId or 0
-		local nodeValue = treeNodes[selectedNode.id].sd[1]
+		local nodeValue = treeNodes[selectedNode.id].sd[1] or ""
 		for id, node in pairs(self.build.spec.tree.tattoo.nodes) do
 			if (nodeName:match(node.targetType:gsub("^Small ", "")) or (node.targetValue ~= "" and nodeValue:match(node.targetValue)) or
 					(node.targetType == "Small Attribute" and (nodeName == "Intelligence" or nodeName == "Strength" or nodeName == "Dexterity"))