Hello Willem,
this could happen, if there is no arrow_border_width set.
Define the arrow_border_width as 0 and the warning message should disapear.

In version 4.x, this problem should not be present.
The developers have rewritten the site.php.
Alternativly, you can change the site.php.
Change Line 108 from
$arrow_style .= (isset($settings->arrow_height) && $settings->arrow_height) ? "line-height: " . (($settings->arrow_height)-($settings->arrow_border_width)) . "px;" : "";
to
$arrow_style .= (isset($settings->arrow_height) && $settings->arrow_height && isset($settings->arrow_border_width) && $settings->arrow_border_width) ? "line-height: " . (($settings->arrow_height)-($settings->arrow_border_width)) . "px;" : "";
I just added an additional query for the border width.
Now the calculation will only happen, if both variables are present.