clear variable location cell in variable panel, if variable class or type changed
if variable class is not local or global, or variable type is not locatable, variable location cell would be empty and read-only
<?xml version='1.0' encoding='utf-8'?>
<PyFile xmlns:xhtml="http://www.w3.org/1999/xhtml" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<variables/>
<globals>
<xhtml:p><![CDATA[]]></xhtml:p>
</globals>
<init>
<xhtml:p><![CDATA[]]></xhtml:p>
</init>
<cleanup>
<xhtml:p><![CDATA[]]></xhtml:p>
</cleanup>
<start>
<xhtml:p><![CDATA[]]></xhtml:p>
</start>
<stop>
<xhtml:p><![CDATA[]]></xhtml:p>
</stop>
</PyFile>