A federal judge dealt a setback to Lockheed Martin Corp.'s proposed $12-billion acquisition of Northrop Grumman Corp., delaying the trial in the government's challenge to the transaction until Sept. 8. U.S. District Judge Emmet Sullivan said the timetable means he probably will not rule until around Christmas. That could be particularly troubling to Northrop, which is in danger of losing key personnel while its future remains in doubt. Company attorneys pushed for a summer trial and a decision by early October. Both Lockheed Martin and Century City-based Northrop said they have no plans to abandon the deal. Separately, the Pentagon praised the F-22 stealth fighter jet Bethesda, Md.-based Lockheed is developing, but delayed a production decision on the $62-billion program until December 1999, under pressure from Congress for additional testing.