The error "items MUST be present if the type is array" is returned since openApi 3.1, for nullable arrays, as the type is now an array.

if (! isset($this->parentSchema->type) || ($this->parentSchema->type !== 'array')) {
            throw new InvalidSchema(sprintf('items MUST be present if the type is array'));
        }

The value of $this->parentSchema->type can be an array with :

• "array"
• "null"