Unsupervised probation for six months with the ability to seal your record could be a layman's way of describing a deferred sentence. If that is what it is, it counts as a conviction, for a non drug related offense, only while you are inside the term of the deferred. Once the six months are successfully completed it ceases to be a conviction.
To be more certain as to whether this applies to you. You should get any paperwork on the case you have to an attorney for his/her opinion.
I do not do mj dispensery licensing, but a misdeanor conviction for a non drug conviction, if that is what this turns out to be might not be fatall to holding this job. It would be up to your employer to decide what risks this exposé him to, and he might wish an attorneys opinion.