What will happen if I am about to finish my probation and just got a Class A theft ticket?

The probation department will report to the prosecutor who will talk with judge about filing a motion to adjudicate guilt. If filed, a warrant will issue & you will need to make bond or sit in jail until the case is finished. The burden of proof is significantly less on a probation violation than to get a conviction on the same case. Or, the court could simply allow the probation to finish or terminate unsatisfactorily.