I found that PolylineBuilder throws the AccessViolationException when the builder is used after multiple Dispatcher.Invoke calls. If I use BeginInvoke instead of Invoke, everything goes fine, so it is the workaround. But I believe you should fix this problem.
I've attached VS solution with simple test add-in (TestAddIn.zip) and test data (test.gdb.zip, place it in Debug folder after solution building). This test add-in contains one button (Click Me!) which opens a dialog. In this dialog click the Start button and you'll get AccessViolationException at line #35 in FeatureClassIterator.cs file.
