Fixes Null References Error from 'new FileModel(null)'

Before fix:
![](http://service.crazypanda.ru/v/clip2net/C/s/suRFeRBeaF.png)

After fix:
![](http://service.crazypanda.ru/v/clip2net/b/E/r1mfgIW7NR.png)

test: https://github.com/SlavaRa/flashdevelop/blob/tests/tests/External/Plugins/ASCompletionTest/Model/FileModelTest.cs#L30-63