:bug: forgot to declare the var first

vue-theme/src/store/modules/artist.js

@@ -18,7 +18,7 @@ const getters = {
 }
 
 const _arrangeByMaterial = artistsList => {
-    byMaterial = {}
+    const byMaterial = {}
     artistsList.forEach(artist => {
         artist.materials.forEach(mat => {
             if(!byMaterial[mat]) byMaterial[mat] = []