Sitecore WFFM save action UI dialog is appearing empty

When creating a Sheer UI dialog for a custom save action in WFFM, it comes up empty like so:

Sitecore WFFM empty save action dialog

It's not mentioned easily in the available examples online, but the location of the XML file for the dialog can cause this error. Inside the Sitecore.config is the following section:

<controlSources>
 <source mode="on" namespace="Sitecore.Web.UI.XmlControls" folder="/sitecore/shell/override" deep="true" />
 <source mode="on" namespace="Sitecore.Web.UI.XmlControls" folder="/layouts" deep="false" />
 <source mode="on" namespace="Sitecore.Web.UI.XmlControls" folder="/sitecore/shell/controls" deep="true" />
 <source mode="on" namespace="Sitecore.Web.UI.XmlControls" folder="/sitecore/shell/applications" deep="true" />
 <source mode="on" namespace="Sitecore.Web.UI.XmlControls" folder="/sitecore modules" deep="true" />
 <source mode="on" namespace="Sitecore.Web.UI.HtmlControls" assembly="Sitecore.Kernel" />
 <source mode="on" namespace="Sitecore.Web.UI.WebControls" assembly="Sitecore.Kernel" />
 <source mode="on" namespace="Sitecore.Shell.Web.UI.WebControls" assembly="Sitecore.Kernel" prefix="shell" />
 <source mode="on" namespace="Sitecore.Shell.Applications.ContentEditor" assembly="Sitecore.Kernel" prefix="content" />
 <source mode="on" namespace="Sitecore.Shell.Web.Applications.ContentEditor" assembly="Sitecore.Kernel" prefix="shell" />
 <source mode="on" namespace="Sitecore.WebControls" assembly="Sitecore.Kernel" />
 <source mode="on" namespace="System.Web.UI.WebControls" assembly="System.Web" prefix="asp" />
 <source mode="on" namespace="System.Web.UI.HtmlControls" assembly="System.Web" prefix="html" />
 <source mode="on" namespace="Sitecore.Web.UI.Portal" assembly="Sitecore.Kernel" />
 <source mode="on" namespace="ComponentArt.Web.UI" assembly="ComponentArt.Web.UI" prefix="ca" />
</controlSources>

You could add your own source location in here, or place your XML file under and existing XML location (such as /sitecore/shell/applications).