strange error for empty slots

19.06 22:21:36 [Server] INFO at net.minecraft.server.v1_5_R3.ThreadServerApplication.run(SourceFile:573)
19.06 22:21:36 [Server] INFO at net.minecraft.server.v1_5_R3.MinecraftServer.run(MinecraftServer.java:410)
19.06 22:21:36 [Server] INFO at net.minecraft.server.v1_5_R3.MinecraftServer.q(MinecraftServer.java:477)
19.06 22:21:36 [Server] INFO at net.minecraft.server.v1_5_R3.DedicatedServer.r(DedicatedServer.java:226)
19.06 22:21:36 [Server] INFO at net.minecraft.server.v1_5_R3.MinecraftServer.r(MinecraftServer.java:513)
19.06 22:21:36 [Server] INFO at org.bukkit.craftbukkit.v1_5_R3.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:345)
19.06 22:21:36 [Server] INFO at org.bukkit.craftbukkit.v1_5_R3.scheduler.CraftTask.run(CraftTask.java:53)
19.06 22:21:36 [Server] INFO at com.github.vsams14.energycraft.CondenseTask.run(CondenseTask.java:9)
19.06 22:21:36 [Server] INFO at com.github.vsams14.energycraft.Condenser.condense(Condenser.java:57)
19.06 22:21:36 [Server] INFO at com.github.vsams14.energycraft.Condenser.hasEmptySpace(Condenser.java:137)
19.06 22:21:36 [Server] INFO at org.bukkit.craftbukkit.v1_5_R3.inventory.CraftInventory.firstEmpty(CraftInventory.java:227)
19.06 22:21:36 [Server] INFO at org.bukkit.craftbukkit.v1_5_R3.inventory.CraftInventory.getContents(CraftInventory.java:54)
19.06 22:21:36 [Server] INFO at org.bukkit.craftbukkit.v1_5_R3.inventory.CraftInventory.getSize(CraftInventory.java:41)
19.06 22:21:36 [Server] INFO java.lang.NullPointerException
19.06 22:21:36 [Server] WARNING Task #24 for EnergyCraft v1.3.0 generated an exception