Fix null reference issue when referencing static field that are defined below another static field.
Fix null reference issue when referencing static field that are defined below another static field.

ye4241/MahApps.Metro,Danghor/MahApps.Metro,p76984275/MahApps.Metro,Jack109/MahApps.Metro,jumulr/MahApps.Metro,Evangelink/MahApps.Metro,xxMUROxx/MahApps.Metro,batzen/MahApps.Metro,chuuddo/MahApps.Metro,psinl/MahApps.Metro,pfattisc/MahApps.Metro,MahApps/MahApps.Metro