lib/Dropbox/AppInfo.php
static function checkArgOrNull($argName, $argValue)
    {
        if ($argValue === null) return;
        if (!($argValue instanceof self)) Checker::throwError($argName, $argValue, __CLASS__);
    }
static function checkArgOrNull($argName, $argValue)
    {
        if ($argValue === null) return;
        if (!($argValue instanceof self)) Checker::throwError($argName, $argValue, __CLASS__);
    }
Use this to check that a function argument is either `null` or of type
`AppInfo`.

@internal