Add own sys_category to record with extbase - typo3

Question

I made the table of one of my models categorizable which works fine if I set categories in the Backend. If I try to add categories via a frontend form I always get the error:
Call to a member function attach() on null
and have no clue why that is so. Maybe anyone of you can help.
In controller I try to add as usual, find the category
$videoCat = $this->categoryRepository->findByUid(28);
and add it like that
$this->video->addTxVideoCat($videoCat);
That's where the error occurs.
Find below how I added the category to the model.
ext_tables.sql `tx_video_cat int(11) DEFAULT '0' NOT NULL,`
extended tca in TCA/Overrides/sys_template.php
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::makeCategorizable(
'addvideo',
'tx_addvideo_domain_model_video',
// Do not use the default field name ("categories") for pages, tt_content, sys_file_metadata, which is already used
'tx_video_cat',
array(
// Set a custom label
'label' => 'LLL:EXT:addvideo/Resources/Private/Language/locallang.xlf:video_categories',
// This field should not be an exclude-field
'exclude' => FALSE,
// Override generic configuration, e.g. sort by title rather than by sorting
// string (keyword), see TCA reference for details
'l10n_mode' => 'exclude',
// list of keywords, see TCA reference for details
'l10n_display' => 'hideDiff',
)
);
Created extended category repository
namespace Pixelink\Addvideo\Domain\Repository;
class CategoryRepository extends \TYPO3\CMS\Extbase\Domain\Repository\CategoryRepository
Created extended model
namespace Pixelink\Addvideo\Domain\Model;
class Category extends \TYPO3\CMS\Extbase\Domain\Model\Category
{}
Category mapping in typoscript
plugin.tx_addvideo{
persistence {
classes{
Pixelink\Addvideo\Domain\Model\Category {
mapping {
tableName = sys_category
columns {
}
}
}
}
}
}
and in Model\Video.php I added the following
/**
* txVideoCat
*
* #var \TYPO3\CMS\Extbase\Persistence\ObjectStorage<\Pixelink\Addvideo\Domain\Model\Category>
*/
protected $txVideoCat = null;
/**
* Get categories
*
* #return \TYPO3\CMS\Extbase\Persistence\ObjectStorage<\Pixelink\Addvideo\Domain\Model\Category>
*/
public function getTxVideoCat()
{
return $this->txVideoCat;
}
/**
* Set categories
*
* #param \TYPO3\CMS\Extbase\Persistence\ObjectStorage $txVideoCat
*/
public function setTxVideoCat($txVideoCat)
{
$this->txVideoCat = $txVideoCat;
}
/**
* Add category to a post
*
* #param \Pixelink\Addvideo\Domain\Model\Category $txVideoCat
*/
public function addTxVideoCat(\Pixelink\Addvideo\Domain\Model\Category $txVideoCat)
{
$this->txVideoCat->attach($txVideoCat);
}

Answer 1

You should initialize your property in your Video model constructor:
public function __construct()
{
$this->txVideoCat = new ObjectStorage();
}

Answer 2

Your $this->txVideoCat is null. Use initiailizeObject() method to assign it:
public function initializeObject()
{
$this->txVideoCat = new ObjectStorage();
}