Unsafe code in MDIManagerImpl.cpp

Сurrently crashes may occur in MDIManagerImpl.cpp due to lack of checking of pointers for nulls. Refactor the code, add additional checks and replace assert() with SAFE_POINT().
For exampale in 399 line there is those code: "mdiContentOwner->w->setupMDIToolbar(tb);", but mdiContentOwner and w are not checked for nulls.
Crash report:
http://ugene.net/ugenecrashes?id=6821