2012-07-08 19:11:43 [SEVERE] null
org.bukkit.command.CommandException: Unhandled exception executing command 'mob' in plugin MobRider v0.45.4-b28
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:42)
at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:166)
at org.bukkit.craftbukkit.CraftServer.dispatchCommand(CraftServer.java:479)
at net.minecraft.server.NetServerHandler.handleCommand(NetServerHandler.java:822)
at net.minecraft.server.NetServerHandler.chat(NetServerHandler.java:782)
at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:765)
at net.minecraft.server.Packet3Chat.handle(Packet3Chat.java:34)
at net.minecraft.server.NetworkManager.b(NetworkManager.java:229)
at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:114)
at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:78)
at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:567)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:459)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)
Caused by: java.lang.NullPointerException
at com.edwardhand.mobrider.models.Rider.(Rider.java:51)
at com.edwardhand.mobrider.utils.MRHandler.addRider(MRHandler.java:30)
at com.edwardhand.mobrider.commands.MountCommand.execute(MountCommand.java:43)
at com.edwardhand.mobrider.commands.CommandHandler.dispatch(CommandHandler.java:81)
at com.edwardhand.mobrider.MobRider.onCommand(MobRider.java:116)
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:40)
... 12 more

This was fixed in a later dev build. Try it out here:
http://ci.bighatchet.com/job/ecoCreature/lastSuccessfulBuild/

Do you mean "MobRider-0.46.0-SNAPSHOT"? I'm using that version at the moment and it still occurs! What to do?

Make sure you delete the old MobRider.jar and then rename MobRider-0.46.0-SNAPSHOT.jar to MobRider.jar.