EMMessage.createSendMessage(EMMessage.Type.TXT) 创建空值

EMConversation conversation = EMChatManager.getInstance().getConversation(userName);
EMMessage message = EMMessage.createSendMessage(EMMessage.Type.TXT);
这个自定义 消息，如果在 server上执行，就会报错
07-08 05:01:38.759: E/AndroidRuntime(4142): java.lang.NullPointerException
07-08 05:01:38.759: E/AndroidRuntime(4142): at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:374)
07-08 05:01:38.759: E/AndroidRuntime(4142): at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:369)
07-08 05:01:38.759: E/AndroidRuntime(4142): at com.easemob.chat.EMSessionManager.getLastLoginUser(Unknown Source)
07-08 05:01:38.759: E/AndroidRuntime(4142): at com.easemob.chat.EMMessage.createSendMessage(Unknown Source)

在activity上使用 相同的接口就是ok的，请确认这个是不是需要满足一定条件才行，谢谢