Added null verification to avoid NullPointer

Added null verification to avoid NullPointer.
Signed-off-by: Bruno Nocera Zanette <brunonzanette@gmail.com>

src/main/java/br/ufpr/c3sl/sapos/models/util/PermissionProvider.java

@@ -223,10 +223,16 @@ public class PermissionProvider {
 		
 		SectionPos sp = SectionPos.findSectioPosOfTranscriptId(id);
 		
+		if ( sp == null 
+				|| currentYear == null 
+				|| currentPeriod == null 
+				|| modificationPeriodIsOpen == null)
+			return false;
+		
 		if ( modificationPeriodIsOpen.equals("true") 
-				& sp.getPeriod().equals(currentPeriod)
-				& sp.getSectionYear().equals(currentYear)
-			) return true;			
+				&& sp.getPeriod().equals(currentPeriod)
+				&& sp.getSectionYear().equals(currentYear))
+			return true;			
 		
 		return false;
 	}