Hiding a button disables ribbon

Hi,

I am trying to hide a button in the PWA ribbon but each time I try it successfully hides but there is an error that causes all the ribbon buttons to be disabled. The same thing happens when trying to hide any button.

Any ideas on what could be going wrong? I am using Project Server 2010.

Here is the code I use to try to hide it: 

<CustomAction
    Id="Ribbon.Tabs.ResourceConstraintAnalysis.Home.Display.AvailReq"
    Location="CommandUI.Ribbon"
    Title="Hides the Requirements Details button">
  <CommandUIExtension>
    <CommandUIDefinitions>
      <CommandUIDefinition Location="Ribbon.Tabs.ResourceConstraintAnalysis.Home.Display.AvailReq"/>
    </CommandUIDefinitions>
  </CommandUIExtension>
</CustomAction>

Here is the error on the page:

"javascript sys.argumentnullexception: value cannot be null"