Java.lang.NullPointerException error occured when executing command snmp4sdn:topodiscoverswitches

Three switches are used in my experiment, and the connection between them is as follows:

s1(GE1/0/1)-----(GE1/0/1)s2(GE1/0/2)-----(GE1/0/1)s3

Firstly, I use command snmp4sdn:readdb to read the device information of s1 and s2, and the detailed information is as follows:

MAC,IP,SNMP_Community,CLI_Username,CLI_Password
06:DF:44:64:01:00,177.0.0.104,public, ,
06:E4:80:8E:02:00,177.0.0.105,public, ,

Then, I use command snmp4sdn:topodiscoverswitches to discovery the two switches. But there is an ERROR log of java.lang.NullPointerException after executing the command snmp4sdn:topodiscoverswitches. The detailed information is as follows:

2015-10-29 20:07:24,671 | INFO | l for user karaf | CmethUtil | 270 - org.opendaylight.snmp4sdn - 0.2.1.Lithium-SR1 | enter CmethUtil.readDB()
2015-10-29 20:07:24,671 | INFO | l for user karaf | CmethUtil | 270 - org.opendaylight.snmp4sdn - 0.2.1.Lithium-SR1 | open file /odl/switch_login_db.csv
2015-10-29 20:07:34,182 | INFO | l for user karaf | Controller | 270 - org.opendaylight.snmp4sdn - 0.2.1.Lithium-SR1 | Add switch(00:00:06:e4:80:8e:02:00) to the Controller
2015-10-29 20:07:34,701 | INFO | l for user karaf | Controller | 270 - org.opendaylight.snmp4sdn - 0.2.1.Lithium-SR1 | Add to switch (ip: 177.0.0.105, mac: 00:00:06:e4:80:8e:02:00) a new port, port number = 2
2015-10-29 20:07:34,703 | INFO | l for user karaf | Controller | 270 - org.opendaylight.snmp4sdn - 0.2.1.Lithium-SR1 | Add to switch (ip: 177.0.0.105, mac: 00:00:06:e4:80:8e:02:00) a new port, port number = 3
2015-10-29 20:07:34,704 | INFO | l for user karaf | Controller | 270 - org.opendaylight.snmp4sdn - 0.2.1.Lithium-SR1 | Add to switch (ip: 177.0.0.105, mac: 00:00:06:e4:80:8e:02:00) a new port, port number = 4
2015-10-29 20:07:34,712 | INFO | l for user karaf | Controller | 270 - org.opendaylight.snmp4sdn - 0.2.1.Lithium-SR1 | Add switch(00:00:06:df:44:64:01:00) to the Controller
2015-10-29 20:07:35,229 | INFO | l for user karaf | Controller | 270 - org.opendaylight.snmp4sdn - 0.2.1.Lithium-SR1 | Add to switch (ip: 177.0.0.104, mac: 00:00:06:df:44:64:01:00) a new port, port number = 2
2015-10-29 20:07:35,229 | INFO | l for user karaf | Controller | 270 - org.opendaylight.snmp4sdn - 0.2.1.Lithium-SR1 | Add to switch (ip: 177.0.0.104, mac: 00:00:06:df:44:64:01:00) a new port, port number = 3
2015-10-29 20:07:35,399 | ERROR | Thread-43 | TopologyServiceShim | 270 - org.opendaylight.snmp4sdn - 0.2.1.Lithium-SR1 |
java.lang.NullPointerException
at org.opendaylight.snmp4sdn.internal.TopologyServiceShim$TopologyNotify.run(TopologyServiceShim.java:138)[270:org.opendaylight.snmp4sdn:0.2.1.Lithium-SR1]
at java.lang.Thread.run(Thread.java:745)[:1.7.0_80]