fix open scurve error

app/Http/Controllers/ProjectController.php

@@ -581,10 +581,12 @@ class ProjectController extends Controller
         $plannedEnd = Activity::where('version_gantt_id', $ganttId)
 				->orderByDesc('planned_end')
 				->value('planned_end');
+		if (isset($result->header)) {
 			$result->header->start_date = $startDate;
 			$result->header->end_date = $maxEndDate->end_date;
 			$result->header->planned_start = $plannedStart;
 			$result->header->planned_end = $plannedEnd;
+		}
 		return response()->json(['status'=>'success','code'=> 200,'data'=>$result, 'gantt'=>$ganttId], 200);
 	}