I'm trying to use "Date Picker" widget as defined here and here but I'm doing something wrong since I get this error:

Trying to get property of non-object
/var/www/html/octproject.dev/modules/backend/classes/FormWidgetBase.php line 113

Type                Exception
Undefined           ErrorException 

107     * supports nesting via HTML array.
108     * @return string
109     */
110    public function getLoadData()
111    {
112        list($model, $attribute) = $this->getModelArrayAttribute($this->valueFrom);
113        return $model->{$attribute};
114    }
115  
116    /**
117     * Returns the final model and attribute name of
118     * a nested HTML array attribute.
119     * Eg: list($model, $attribute) = $this->getModelArrayAttribute($this->valueFrom);

This is what I'm doing for add the field on the plugin file:

UsersController::extendFormFields(function ($form, $model, $context) {
        if (!$model instanceof UserModel)
            return;

        if (!$model->exists)
            return;

        // Ensures that a profile model always exists ...
        ProfileModel::getFromUser($model);

        $form->addTabFields([
                'profile[pin]' => [
                    'label' => 'User PIN',
                    'tab' => 'Profile',
                    'type' => 'text'
                ],
                'profile[pinRegeneratedDateTime]' => [
                    'label' => 'PIN set on',
                    'tab' => 'Profile',
                    'type' => 'Backend\FormWidgets\DatePicker', // tried with the registered alias name datepicker without success
                    'mode' => 'date'
                ],
                'profile[phone2]' => [
                    'label' => 'Teléfono(2)',
                    'tab' => 'Profile',
                    'type' => 'text'
                ],
                'profile[phone3]' => [
                    'label' => 'Teléfono(3)',
                    'tab' => 'Profile',
                    'type' => 'text'
                ],
                'profile[phone4]' => [
                    'label' => 'Teléfono(4)',
                    'tab' => 'Profile',
                    'type' => 'text'
                ]
            ]);
    });

I'm doing something wrong? Or this is not supported?

This error means the "profile" relation does not exist. It needs to be created otherwise the object reference is null.