Fix NPE related to BlockDispenseEntityEvent

[cswhite2000]

NPE:

java.lang.NullPointerException: Cannot invoke "net.minecraft.server.v1_8_R3.Entity.getBukkitEntity()" because "entity" is null
    at net.minecraft.server.v1_8_R3.DispenserRegistry$6.b(DispenserRegistry.java:93)
    at net.minecraft.server.v1_8_R3.DispenseBehaviorItem.a(DispenseBehaviorItem.java:13)
    at net.minecraft.server.v1_8_R3.BlockDispenser.dispense(BlockDispenser.java:86)
    at net.minecraft.server.v1_8_R3.BlockDispenser.b(BlockDispenser.java:115)
    at net.minecraft.server.v1_8_R3.WorldServer.a(WorldServer.java:683)
    at net.minecraft.server.v1_8_R3.WorldServer.doTick(WorldServer.java:247)
    at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:941)
    at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:385)
    at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:827)
    at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:728)
    at java.base/java.lang.Thread.run(Thread.java:1583)