Previously stored .grid files does not open in later versions of MP, added "laps" xml-parameter

#### Issue details
_Please describe the problem, or desired feature_
Hi!
Previously stored .grid files does not open in latest updated version of MP:

There is an issue with the xml definition, by which a new parameter is added:
< laps >200< /laps >

If this is not available in the previous files, it will not open. Previous versions will open if this is added.

The error report is shown below:

Thanks and best regards

Reproduce the failure by opening a previously stored .grid file in the latest Mission Planner version, then inspect the XML-loading path implicated by the reported error. Compare files with and without the laps parameter; done means older .grid files open successfully without manual editing.

Written by the indexing model from the issue text.