Menu Error / order form not displaying drop down

Hi There :laugh:

I was wondering if anyone could help, please?
When I try to click validate on the create order form, the drop-down menu for selecting the warehouse to decrease stock from is displaying incorrectly, it is blank and the borders are incomplete. (see pic please) I can use the arrow keys to navigate the list blindly but cant actually see the text until after I have selected one, finding the correct one is quite hard to do as I have a lot of warehouse spaces.
Is this an error, like is the size of the pop-up box wrong or have i done/not done something to cause it?

any help would be muchly appreciated :woohoo:
Regards

Sean