try this

Author: jwklong

src/addons/addons/reorder-custom-inputs/userscript.js

@@ -100,15 +100,16 @@ export default async function ({ addon, console }) {
 
     const original = procedureBlock.inputList.find((input) => input.name === inputNameToShift);
     const originalPosition = procedureBlock.inputList.findIndex((input) => input.name === inputNameToShift);
-    const itemToMove = procedureBlock.inputList.splice(originalPosition, 1)[0];
 
     if (
       (newPosition == 0 && original.type === 3/*Blockly.NEXT_STATEMENT*/) ||
-      (originalPosition == 0 && itemToMove.type === 3/*Blockly.NEXT_STATEMENT*/)
+      (originalPosition == 0 && procedureBlock.inputList[newPosition].type === 3/*Blockly.NEXT_STATEMENT*/)
     ) {
       return false
     }
 
+    const itemToMove = procedureBlock.inputList.splice(originalPosition, 1)[0];
+
     procedureBlock.inputList.splice(newPosition, 0, itemToMove);
 
     Blockly.Events.disable();