fix: new program editing exception

src/views/screen/material/program/ast/index.vue

@@ -55,7 +55,7 @@ export default {
               return
             }
 
-            const widget = JSON.parse(itemJsonStr)
+            const widget = itemJsonStr ? JSON.parse(itemJsonStr) : null
 
             this.program = {
               id, name, resolutionRatio,