Render non-primitive extension values more reliably

src/core/components/operation-extension-row.jsx

@@ -2,9 +2,11 @@ import React from "react"
 import PropTypes from "prop-types"
 
 export const OperationExtRow = ({ xKey, xVal }) => {
+  const xNormalizedValue = !xVal ? null : xVal.toJS ? xVal.toJS() : xVal
+
     return (<tr>
         <td>{ xKey }</td>
-        <td>{ String(xVal) }</td>
+        <td>{ JSON.stringify(xNormalizedValue) }</td>
     </tr>)
 }
 OperationExtRow.propTypes = {