tdf#138507 error in loading form

regression from
    commit 3de38e95561ab7ca114d9f3307702ba89c4e3e9a
    Author: Noel Grandin <noelgrandin@gmail.com>
    Date:   Tue Nov 10 19:20:06 2020 +0200
    use fastparser in forms

Change-Id: I995677d4e381e8229e7f052cf54a134bdf49d378
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/106708
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
(cherry picked from commit 3faf503b1a389affcff1bcbd63630963f634e486)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/106734
Reviewed-by: Xisco Fauli <xiscofauli@libreoffice.org>
diff --git a/xmloff/source/forms/elementimport.cxx b/xmloff/source/forms/elementimport.cxx
index 0b14b31..896aaf3 100644
--- a/xmloff/source/forms/elementimport.cxx
+++ b/xmloff/source/forms/elementimport.cxx
@@ -140,7 +140,7 @@ namespace xmloff
    {
        ENTER_LOG_CONTEXT( "xmloff::OElementImport - importing one element" );

        const OUString sControlImplementation = _rxAttrList->getValue( XML_ELEMENT(FORM, XML_CONTROL_IMPLEMENTATION) );
        const OUString sControlImplementation = _rxAttrList->getOptionalValue( XML_ELEMENT(FORM, XML_CONTROL_IMPLEMENTATION) );

        // retrieve the service name
        if ( !sControlImplementation.isEmpty() )