title: Disabling fields on Bills and Adjustments doesn't seem to work
body: <p>I have a custom dropdown field on Bills and Adjustments which I want to determine when to disable specific fields on the screen.  I'm using the following logic, which doesn't seem to work (the commented lines didn't work either).  I've set the commitchanges to true on the user field - and I've stepped through the code to make sure it's getting hit:</p>
  
  <pre><code>    protected virtual void APInvoice_UsrPOStatus_FieldUpdated(PXCache sender, PXFieldUpdatedEventArgs e)
      {
          var apr = (APRegister)e.Row;
          if (apr == null) return;
  
          var aprext = PXCache&lt;APRegister&gt;.GetExtension&lt;APRegisterExt&gt;(apr);
          if (aprext == null) return;
  
          if (aprext.UsrPOstatus != "Open")
          {
              PXUIFieldAttribute.SetEnabled&lt;APRegister.docType&gt;(sender, apr, false);
              PXUIFieldAttribute.SetEnabled&lt;APRegister.refNbr&gt;(sender, apr, false);
              //PXUIFieldAttribute.SetEnabled&lt;APInvoice.docType&gt;(Base.Document.Cache, null, false);  //(OpenSourceDataDetail.Cache, null, true);
              //PXUIFieldAttribute.SetEnabled&lt;APInvoice.refNbr&gt;(Base.Document.Cache, null, false);
          }
      }
  </code></pre>
  
  <p>I get no errors, but nothing happens.  Is it not possible to disable these fields?</p>
  
  <p>I'm also not sure whether to use APInvoice or APRegister for these statements.</p>