Fixes Null References Error from TraceManager.Add(null);

Before:
![](http://service.crazypanda.ru/v/clip2net/h/2/67WyazEpyy.png)

After fix:
![](http://service.crazypanda.ru/v/clip2net/P/u/NcPJJui8n4.png)

test: https://github.com/SlavaRa/flashdevelop/blob/tests/tests/PluginCore.Test/PluginCore/Managers/TraceManagerTest.cs#L12-54