Issues with naming image files "null" as in e.g. null.mrc

[acardona]

As reported by Richard Fetter (thank you!)

"My samples were a 'Control' and a 'Null' .... I had embedded 'Null' in the filename for the MRC stack for each 'null' mutant. I extracted each MRC image from the stack normally but each separate MRC file had 'Null' in the filename. Apparently TrakEM2 will import the file without problem the first time, but subsequent opening of a montage involves loading the XML file, and having 'Null' in the filename screws up TrakEM2."

Looks like an issue in the XML parser.