cannot crawl with inject

[Dan.Wu]

Hi,

I am new for both nutch and mongodb. Now I have installed nutch 2.3 and mongodb 
3.0.7 on Ubuntu 14.0.4. I created nutch database in mongodb, when I run 
bin/nutch inject urls/seed.txt, the following error message come. many thanks 
for your help!

InjectorJob: starting at 2015-12-01 10:52:18
InjectorJob: Injecting urlDir: urls/seed.txt
InjectorJob: org.apache.gora.util.GoraException: java.lang.NullPointerException
        at 
org.apache.gora.store.DataStoreFactory.createDataStore(DataStoreFactory.java:169)
        at 
org.apache.gora.store.DataStoreFactory.createDataStore(DataStoreFactory.java:137)
        at 
org.apache.nutch.storage.StorageUtils.createWebStore(StorageUtils.java:78)
        at org.apache.nutch.crawl.InjectorJob.run(InjectorJob.java:218)
        at org.apache.nutch.crawl.InjectorJob.inject(InjectorJob.java:252)
        at org.apache.nutch.crawl.InjectorJob.run(InjectorJob.java:275)
        at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
        at org.apache.nutch.crawl.InjectorJob.main(InjectorJob.java:284)
Caused by: java.lang.NullPointerException
        at 
org.apache.gora.mongodb.store.MongoStore.initialize(MongoStore.java:173)
        at 
org.apache.gora.store.DataStoreFactory.initializeDataStore(DataStoreFactory.java:104)
        at 
org.apache.gora.store.DataStoreFactory.createDataStore(DataStoreFactory.java:163)
        ... 7 more

Regards/Dan