PHP（Laravel）问题

I am receiving an error from running the below code?

(1/1) ErrorException
Creating default object from empty value

Code:

public function setServiceSetting(Request $request) {
    if (!$request->has('setting_key') ||
        !$request->has('setting_value')) {
        return $this->getScriptingResponse();
    }

    $settingKey = $request->input('setting_key');
    $settingValue = $request->input('setting_value');

    $settings = WebsiteSettings::first();

    if ($settings == null) {
        return;
    }

    $setting->$settingKey = $settingValue;
    $settings->save();
}

Error Line:

$setting->$settingKey = $settingValue;

I know the column exists, below anyone suggests that. I also know both parameters are non-null.

I think that this is incorrect

$settings->$settingKey = $settingValue;

try this:

$settings->settingKey = $settingValue;