fix(editor): fix undo for URL fields that initialize as null

refs #113

lib/editor/components/EditorInput.js

@@ -132,7 +132,7 @@ export default class EditorInput extends Component {
       case 'POSITIVE_NUM':
       case 'GTFS_BLOCK':
       case 'GTFS_FARE':
-      case 'GTFS_SERVICE':
+      case 'GTFS_SERVICE': {
         // Ensure that an empty string is passed to value prop, so that on 'undo'
         // the input value will revert back to a blank field.
         const stringValue = typeof fieldProps.value === 'undefined'
@@ -144,11 +144,17 @@ export default class EditorInput extends Component {
             <FormControl {...fieldProps} value={stringValue} />
           </FormGroup>
         )
-      case 'URL':
+      }
+      case 'URL': {
+        // Ensure that an empty string is passed to value prop, so that on 'undo'
+        // the input value will revert back to a blank field.
+        const stringValue = typeof fieldProps.value === 'undefined'
+          ? ''
+          : fieldProps.value
         const elements = [
           <FormGroup {...formProps} key={`${editorField}-input`}>
             {basicLabel}
-            <FormControl {...fieldProps} />
+            <FormControl {...fieldProps} value={stringValue} />
           </FormGroup>
         ]
         if (field.name === 'agency_branding_url' || field.name === 'route_branding_url') {
@@ -174,6 +180,7 @@ export default class EditorInput extends Component {
           )
         }
         return <span>{elements}</span>
+      }
       case 'GTFS_ZONE':
         return (
           <FormGroup {...formProps}>