Correctly handle nulls in ListBlock validation errors

Fixes @cnk's test case from #7248. blockErrors within a ListBlockValidationError is an array (with nulls for the items with no errors), but setError was treating it as a dict keyed by block index, which meant it was erroneously passing nulls to the child setError method. FieldBlock handles this gracefully, but other blocks such as StructBlock don't.

client/src/components/StreamField/blocks/ListBlock.js

@@ -130,12 +130,13 @@ export class ListBlock extends BaseSequenceBlock {
     }
     const error = errorList[0];
 
-    // eslint-disable-next-line no-restricted-syntax
-    for (const blockIndex in error.blockErrors) {
-      if (error.blockErrors.hasOwnProperty(blockIndex)) {
-        this.children[blockIndex].setError(error.blockErrors[blockIndex]);
+    // error.blockErrors = a list with the same length as the data,
+    // with nulls for items without errors
+    error.blockErrors.forEach((blockError, blockIndex) => {
+      if (blockError) {
+        this.children[blockIndex].setError(blockError);
       }
-    }
+    });
   }
 }
 

client/src/components/StreamField/blocks/ListBlock.test.js

@@ -1,6 +1,6 @@
 /* eslint-disable @typescript-eslint/no-unused-vars */
 
-import { FieldBlockDefinition } from './FieldBlock';
+import { FieldBlock, FieldBlockDefinition } from './FieldBlock';
 import { ListBlockDefinition, ListBlockValidationError } from './ListBlock';
 
 import $ from 'jquery';
@@ -43,6 +43,26 @@ class ValidationError {
   }
 }
 
+
+/* ListBlock should not call setError on its children with a null value; FieldBlock handles this
+gracefully, so define a custom one that doesn't
+*/
+
+class ParanoidFieldBlock extends FieldBlock {
+  setError(errorList) {
+    if (!errorList) {
+      throw new Error('ParanoidFieldBlock.setError was passed a null errorList');
+    }
+    return super.setError(errorList);
+  }
+}
+
+class ParanoidFieldBlockDefinition extends FieldBlockDefinition {
+  render(placeholder, prefix, initialState, initialError) {
+    return new ParanoidFieldBlock(this, placeholder, prefix, initialState, initialError);
+  }
+}
+
 describe('telepath: wagtail.blocks.ListBlock', () => {
   let boundBlock;
 
@@ -57,7 +77,7 @@ describe('telepath: wagtail.blocks.ListBlock', () => {
     // Define a test block
     const blockDef = new ListBlockDefinition(
       'test_listblock',
-      new FieldBlockDefinition(
+      new ParanoidFieldBlockDefinition(
         '',
         new DummyWidgetDefinition('The widget'),
         {