Fix modelManager API usage

Our testing team found a weird issue where the contents of an XML was re-appearing even after the file had been deleted, the old contents of the file were being loaded from memory.  We I've seen the problem before it can always be traced back to where a call to modelManger getModelForRead/Edit() but the cooresponding model.releaseFromRead/Write() wasn't called.

I debugged it and traced it down to LineStyleProviderForXMLReferences class.  This pull fixes that issue.