Unsafe code in DocumentModel.cpp

Сurrently crashes may occur in DocumentModel.cpp due to lack of checking of pointers for nulls. Refactor the code and add additional checks for nulls.

For exampale in 300 line there is those code: "obj->setParentStateLockItem(this);", but obj can be null, if some errors occur on loading the document.

Crash report:
http://ugene.net/ugenecrashes?id=6822