Single Archive DAT processing options crash with SuperDATs

The Single Archive DAT processing options can crash RomVault with an "Unable to cast object of type" error.

Steps to reproduce:

1. Add the TOSEC-Rollback dat from DatVault

2. Change the DAT settings to Single Archive + Use subdirs for all sets

3. Save the dat rule and notice RV crashes

Example DAT Rule

<DatRule>
  <DirKey>RomVault\Various\Rollbacks\TOSEC</DirKey>
  <DirPath></DirPath>
  <Compression>Zip</Compression>
  <CompressionOverrideDAT>false</CompressionOverrideDAT>
  <Merge>None</Merge>
  <Filter>KeepAll</Filter>
  <HeaderType>Optional</HeaderType>
  <MergeOverrideDAT>false</MergeOverrideDAT>
  <SingleArchive>true</SingleArchive>
  <SubDirType>KeepAllSubDirs</SubDirType>
  <MultiDATDirOverride>false</MultiDATDirOverride>
  <UseDescriptionAsDirName>true</UseDescriptionAsDirName>
  <UseIdForName>false</UseIdForName>
  <CompleteOnly>false</CompleteOnly>
  <IgnoreFiles />
</DatRule>