Caught while reviewing 18323.
Sometimes $oQuestion->survey is throwing an error.

models/QuestionTypes.php

    public function applyToQuestion($oQuestion)
    {
        $this->question = $oQuestion;
        $aSettingsArray = self::modelsAttributes($oQuestion->survey->language)[$oQuestion->type];
        foreach ($aSettingsArray as $settingKey => $setting) {
            $this->$settingKey = $setting;
        }
    }

Sometimes, when creating a question, wrongly, it may happen that the survey is not ready associated to a question.

• The CopyQuestion service creates the question and sub-questions by directly setting the "attributes" property. (that was fixed on 18323, undo it to trigger the error please :) )
• That triggers the execution of rules() because it has to determine the safe attributes.
• Since rules() is executed before the attributes are set, when it gets to applyToQuestion() and it tries to access $oQuestion->survey->language, the relationship with Survey doesn't work because the SID is still 0.

Fix:

• If survey is not set, use null as language to be sent as parameter.