Fixed some issues introduced in last changesets

This patch should increase findbugs report and solve some issues that
will occure:

* Doing operation on NULL object
* Using wrong object type in hope of magic object casting ;-)

src/main/java/de/thm/arsnova/dao/CouchDBDao.java

@@ -1071,15 +1071,14 @@ public class CouchDBDao implements IDatabaseDao {
 	
 	public Question getInterposedQuestion(String sessionKey, String documentId) {
 		try {
-			
 			Document document = this.getDatabase().getDocument(documentId);
-			LOGGER.error("bla test" + document.toString());
-			if(document != null) {
-				Question question = (Question) JSONObject.toBean(document.getJSONObject(), Question.class);
-				question.setQuestionType("interposed_question");
-				
-				return question;
+			if (document == null) {
+				LOGGER.error("Document is NULL");
+				return null;
 			}
+			Question question = (Question) JSONObject.toBean(document.getJSONObject(), Question.class);
+			question.setQuestionType("interposed_question");
+			return question;
 		} catch (IOException e) {
 			LOGGER.error("Error while retrieving interposed question", e);
 		}

src/main/java/de/thm/arsnova/services/UserService.java

@@ -134,9 +134,12 @@ public class UserService implements IUserService, InitializingBean, DisposableBe
 
 	@Override
 	public String getSessionForUser(String username) {
-		return user2session.get(username);
+		for (Entry<User, String> entry  : user2session.entrySet()) {
+			if (entry.getKey().getUsername().equals(username)) return entry.getValue();
+		}
+		return null;
 	}
-
+	
 	@Override
 	public void afterPropertiesSet() {
 		try {