Unhandled Exception: Index was out of range. Fix

Fix maca134#7

Why:
now=100
nextRun = 5
nextRun + 1 = 6
nextRun-now=-94 (Index out of range)

Now:
nextRun = now+next
nextRun - now (Conversion from DateTime)

src/Plugins/ScheduledTasks/Task.cs

@@ -342,8 +342,9 @@ public TimeSpan TimeSpan
             {
                 var now = DateTime.Now;
 
-                if (now > _nextRun)
-                    _nextRun.Add(_interval);
+                if (now > _nextRun) {
+                    _nextRun = now.Add(_interval);
+                }
                 return _nextRun.Subtract(DateTime.Now);
             }
         }