Have to I lock every reference of dictionary?

Question

I have dictionary:

private static Dictionary<string, List<Event>> eventsForArea;

initialize it in static constructor.

And I have method (I only insert part of the method where I have references to dictionary, but I think it is enough to understand my question.):

    public void Analyze()
    {

        if (!eventsForArea.ContainsKey(Area.Name))
        {                
            lock (eventsForArea)
                eventsForArea.Add(Area.Name, allUnfinished);

            var unknownUnfinished = ..;

            foreach (var @event in unknownUnfinished)
            {                  
                eventsForArea[Area.Name].Remove(@event);
            }
        }
        else
        {
            unfinished = eventsForArea[Area.Name].ToArray();
        }


        foreach (var @event in Events)
        {
            try
            {
                var unfinishedEventsForAsixId = ..;
                if (!@event.CheckIfPresent(registers))
                {
                    foreach (var unfinishedEvent in unfinishedEventsForAsixId)
                    {                            
                        eventsForArea[Area.Name].Remove(unfinishedEvent);
                    }
                }
                else
                {                       
                    eventsForArea[Area.Name].Add(eventWithId);
                }
            }
            catch (Exception e)
            {                
            }
        }
    }

The startup method is asynchronous, it may be run multiple times at the same time. Without lock (eventsForArea) it throw exception:

"Object reference not set to an instance of an object"

Now work, but I dont sure is it enough that there is a lock only in this place?