foreach() argument must be of type array|object, null given Yii2

foreach() argument must be of type array|object, null given Yii2, gives error 500 when i click view button

$variableConfig = Yii::$app->session->get($this->createModel->getKeyVariables());
        $browserVariables = [];
        $columns = [];

    if($entity != 'activity'){
                array_push($browserVariables, [
                    'abbrev' => 'DESIGNATION',
                    'name' => 'activity',
                    'label' => 'activity'
                ]);
            }
    
    //Error foreach here gives me error 500, thank you
    
            foreach($variableConfig AS $variable){
                if( $variable['entity'] == $entity && $variable['type'] != 'attribute' && (
                    $variable['entity'] != 'activity' || ($variable['entity'] == 'activity' && $variable['type'] != 'attribute'))){
                    array_push($browserVariables, [
                        'abbrev' => $variable['abbrev'],
                        'name' => $variable['variable_info']['name'],
                        'label' => $variable['variable_info']['label']
                    ]);
                }
            }