Resolve #3502658 "Random test failure"

Closes #3502658

core/modules/views_ui/tests/src/FunctionalJavascript/FilterEntityReferenceTest.php

@@ -110,6 +110,7 @@ public function testAddEntityReferenceFieldWithDefaultSelectionHandler(): void {
     // Test the exposed filter options show up correctly.
     $assert->waitForField('options[expose_button][checkbox][checkbox]');
     $page->findField('options[expose_button][checkbox][checkbox]')->click();
+    $this->assertSession()->assertWaitOnAjaxRequest();
     $this->assertTrue($page->hasCheckedField('options[expose_button][checkbox][checkbox]'));
 
     // Check the exposed filters multiple option.
@@ -210,6 +211,7 @@ public function testAddConfigEntityReferenceFieldWithDefaultSelectionHandler():
     // Test the exposed filter options show up correctly.
     $assert->waitForField('options[expose_button][checkbox][checkbox]');
     $page->findField('options[expose_button][checkbox][checkbox]')->click();
+    $this->assertSession()->assertWaitOnAjaxRequest();
     $this->assertTrue($page->hasCheckedField('options[expose_button][checkbox][checkbox]'));
 
     // Check the exposed filters multiple option.