Magento 2 does not check whether the array key class exists in the array:
$attributes = $data['attributes'];
if (!empty($attributes['group'])) {
$structure->addToParentGroup($elementName, $attributes['group']);
}
if (!empty($attributes['display'])) {
$structure->setAttribute($elementName, 'display', $attributes['display']);
}
// create block
$className = $attributes['class'];
$block = $this->createBlock($className, $elementName, [
'data' => $this->evaluateArguments($data['arguments'])
]);
if (!empty($attributes['template'])) {
$block->setTemplate($attributes['template']);
}
if (!empty($attributes['ttl'])) {
$ttl = (int)$attributes['ttl'];
$block->setTtl($ttl);
}
I my case the cause of the failure was my wrong referenceContainer tag instead of referenceBlock.
I have same issue, anybody help!!!
You should read carefully the last sentence of the acrticle.