Cracks like the one you have are not that uncommon and may have more than one cause. Simple first, caused by the wood drying and shrinking a little. The lock plate does not give causing a surface split. Another cause could be a flaw in the wood grain that can give way caused by the woods natural movement due to climate changes.
My first attempt for a cure would be the simplest first, try to close the crack by binding with strips of bicycle tyre innertube , you would be surprised how much force you can exert.
If the crack closes use a good quality clear Epoxy Resin Adhesive pulled in to the full depth of the crack by using fishing mono filament line or dental floss then re bind with the innertube strips again.
After the adhesive has set it would be a good idea to check the fit of the lock plate is not to tight.
Natural weaknesses in the grain can be a problem that can in many cases come back to haunt you months later and the only method I have found to stop a crack continuing on its relentless journey is to drill a small deep hole at the end of the crack to relieve the internal stresses. To fill the hole I normally remove a little timber from under the trigger guard strap to fill the hole I have made.
Hope this is of some help.
