src/gii/Generator.php
public function validateNamespace($attribute)
    {
        $value = $this->$attribute;
        $value = ltrim($value, '\\');
        $path = Yii::getAlias('@' . str_replace('\\', '/', $value), false);
        if ($path === false) {
            $this->addError($attribute, 'Namespace must be associated with an existing directory.');
        }
    }
public function validateNamespace($attribute)
    {
        $value = $this->$attribute;
        $value = ltrim($value, '\\');
        $path = Yii::getAlias('@' . str_replace('\\', '/', $value), false);
        if ($path === false) {
            $this->addError($attribute, 'Namespace must be associated with an existing directory.');
        }
    }
Validates the namespace.

@param string $attribute Namespace attribute name.