🐛 [#5088] Fix backendOptions not available error in registration vari…

…ables table of JSON dump plugin

If the JSON dump plugin was not completely initialized yet (selected but not saved), and the user navigated to the registration variables of the variables table, the backend options would be an empty object, causing a crash.

src/openforms/js/components/admin/form_design/registrations/json_dump/JSONDumpSummaryHandler.js

@@ -5,10 +5,12 @@ import {FormattedMessage} from 'react-intl';
 import {IconNo, IconYes} from 'components/admin/BooleanIcons';
 
 const JSONDumpSummaryHandler = ({variable, backendOptions}) => {
-  const isIncludedInVariables = backendOptions.variables.includes(variable.key);
+  const isIncludedInVariables = backendOptions.variables?.includes(variable.key);
+  // TODO-5087: is it OK to get the fixed variables from the backend options, or better to always
+  //  use the API for it?
   const isIncludedInMetadata =
-    backendOptions.fixedMetadataVariables.includes(variable.key) ||
-    backendOptions.additionalMetadataVariables.includes(variable.key);
+    backendOptions.fixedMetadataVariables?.includes(variable.key) ||
+    backendOptions.additionalMetadataVariables?.includes(variable.key);
 
   return (
     <>