Cannot convert null literal to non-nullable reference type.

Non-nullable field '_downloadTasks' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the field as nullable.

Non-nullable event 'MultiAllDownloadCompleted' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the event as nullable.

Non-nullable event 'MultiDownloadCompleted' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the event as nullable.

Non-nullable event 'MultiDownloadError' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the event as nullable.

Non-nullable event 'MultiDownloadStatistics' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the event as nullable.

Possible null reference argument for parameter 'value' in 'void Dictionary<string, object>.Add(string key, object value)'.

Possible null reference argument for parameter 'item' in 'bool Builder.Contains(DownloadTask item)'.

Possible null reference return.