Wix: show custom dialog if previous version found

I want to customize my installer to show custom dialog when previous version is already installed: after Welcome dialog user should see a custom dialog OldVersionDlg with information that previous version was found and will be uninstalled automatically.

But for some reason property set by UpgradeVersion element always null when I check it in condition in UI/Publish Dialog.

Here are essential code snippets.

Product.wxs:

<Product Id="*" Version="$(var.Version)" UpgradeCode="$(var.ProductId)"
         Language="1033" Name="$(var.ProductFullName)" Manufacturer="$(var.Manufacturer)">
  <Package Description="$(var.ProductDescription)" InstallerVersion="200" Compressed="yes" 
           Manufacturer="$(var.Manufacturer)" />

  <Property Id="PREVIOUSVERSIONSINSTALLED" Secure="yes" />
  <Upgrade Id="$(var.ProductId)">
    <UpgradeVersion Minimum="1.0.0.0" Maximum="$(var.Version)"
                    Property="PREVIOUSVERSIONSINSTALLED"
                    IncludeMinimum="yes" IncludeMaximum="no" />
  </Upgrade>

  <InstallExecuteSequence>
    <RemoveExistingProducts Before="InstallInitialize" />
  </InstallExecuteSequence>
</Product>

WixUI_Wizard.wxs:

<Publish Dialog="WelcomeDlg" Control="Next" Event="NewDialog" Value="OldVersionDlg">PREVIOUSVERSIONSINSTALLED</Publish>
<Publish Dialog="WelcomeDlg" Control="Next" Event="NewDialog" Value="SetupTypeDlg">NOT Installed</Publish>

The button Next doesn't work. I've checked in logs that PREVIOUSVERSIONSINSTALLED is set after FindRelatedProducts. If I use it in conditions in Product.wxs then everything is OK. But in UI configuration it is always null.

Thanks for any help.

The problem was caused by the second line in WixUI_Wizard.wxs. For some reason WiX always uses it. So, to implement checking of previous version we need to exclude PREVIOUSVERSIONSINSTALLED from the second condition:

<Publish Dialog="WelcomeDlg" Control="Next" Event="NewDialog" Value="OldVersionDlg">PREVIOUSVERSIONSINSTALLED</Publish>
<Publish Dialog="WelcomeDlg" Control="Next" Event="NewDialog" Value="SetupTypeDlg">NOT Installed AND NOT PREVIOUSVERSIONSINSTALLED</Publish>