Added protection against exceptions when sorting by special field columns.
Added protection against exceptions when sorting by special field columns.

shitikanth/jabref,grimes2/jabref,tschechlovdev/jabref,tschechlovdev/jabref,ayanai1/jabref,ayanai1/jabref,ayanai1/jabref,oscargus/jabref,tschechlovdev/jabref,Siedlerchr/jabref,bartsch-dev/jabref,jhshinn/jabref,mairdl/jabref,Siedlerchr/jabref,shitikanth/jabref,zellerdev/jabref,obraliar/jabref,tschechlovdev/jabref,obraliar/jabref,JabRef/jabref,Siedlerchr/jabref,jhshinn/jabref,motokito/jabref,motokito/jabref,sauliusg/jabref,mredaelli/jabref,zellerdev/jabref,obraliar/jabref,Braunch/jabref,obraliar/jabref,sauliusg/jabref,sauliusg/jabref,oscargus/jabref,grimes2/jabref,bartsch-dev/jabref,mairdl/jabref,bartsch-dev/jabref,oscargus/jabref,tobiasdiez/jabref,tobiasdiez/jabref,mredaelli/jabref,Braunch/jabref,Braunch/jabref,jhshinn/jabref,bartsch-dev/jabref,grimes2/jabref,zellerdev/jabref,zellerdev/jabref,jhshinn/jabref,tobiasdiez/jabref,mredaelli/jabref,Mr-DLib/jabref,jhshinn/jabref,Mr-DLib/jabref,Mr-DLib/jabref,JabRef/jabref,mairdl/jabref,Mr-DLib/jabref,sauliusg/jabref,Mr-DLib/jabref,ayanai1/jabref,tschechlovdev/jabref,oscargus/jabref,shitikanth/jabref,JabRef/jabref,motokito/jabref,zellerdev/jabref,grimes2/jabref,bartsch-dev/jabref,mredaelli/jabref,motokito/jabref,motokito/jabref,Braunch/jabref,shitikanth/jabref,shitikanth/jabref,tobiasdiez/jabref,grimes2/jabref,oscargus/jabref,mairdl/jabref,mredaelli/jabref,JabRef/jabref,Braunch/jabref,obraliar/jabref,mairdl/jabref,ayanai1/jabref,Siedlerchr/jabref