[15:58:45 WARN]: Exception in thread "Craft Scheduler Thread - 6"
[15:58:45 WARN]: org.apache.commons.lang.UnhandledException: Plugin Shopkeepers v1.63-SNAPSHOT generated an exception while executing task 1650
        at org.bukkit.craftbukkit.v1_8_R3.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:56)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException: null value in entry: skull-owner=null
        at com.google.common.collect.CollectPreconditions.checkEntryNotNull(CollectPreconditions.java:33)
        at com.google.common.collect.ImmutableMap.entryOf(ImmutableMap.java:135)
        at com.google.common.collect.ImmutableMap$Builder.put(ImmutableMap.java:206)
        at org.bukkit.craftbukkit.v1_8_R3.inventory.CraftMetaSkull.serialize(CraftMetaSkull.java:173)
        at org.bukkit.craftbukkit.v1_8_R3.inventory.CraftMetaItem.serialize(CraftMetaItem.java:850)
        at org.bukkit.configuration.file.YamlRepresenter$RepresentConfigurationSerializable.representData(YamlRepresenter.java:33)
        at org.yaml.snakeyaml.representer.BaseRepresenter.representData(BaseRepresenter.java:94)
        at org.yaml.snakeyaml.representer.BaseRepresenter.representMapping(BaseRepresenter.java:156)
        at org.yaml.snakeyaml.representer.SafeRepresenter$RepresentMap.representData(SafeRepresenter.java:304)
        at org.bukkit.configuration.file.YamlRepresenter$RepresentConfigurationSerializable.representData(YamlRepresenter.java:35)
        at org.yaml.snakeyaml.representer.BaseRepresenter.representData(BaseRepresenter.java:94)
        at org.yaml.snakeyaml.representer.BaseRepresenter.representMapping(BaseRepresenter.java:156)
        at org.yaml.snakeyaml.representer.SafeRepresenter$RepresentMap.representData(SafeRepresenter.java:304)
        at org.bukkit.configuration.file.YamlRepresenter$RepresentConfigurationSection.representData(YamlRepresenter.java:23)
        at org.yaml.snakeyaml.representer.BaseRepresenter.representData(BaseRepresenter.java:94)
        at org.yaml.snakeyaml.representer.BaseRepresenter.representMapping(BaseRepresenter.java:156)
        at org.yaml.snakeyaml.representer.SafeRepresenter$RepresentMap.representData(SafeRepresenter.java:304)
        at org.bukkit.configuration.file.YamlRepresenter$RepresentConfigurationSection.representData(YamlRepresenter.java:23)
        at org.yaml.snakeyaml.representer.BaseRepresenter.representData(BaseRepresenter.java:94)
        at org.yaml.snakeyaml.representer.BaseRepresenter.representMapping(BaseRepresenter.java:156)
        at org.yaml.snakeyaml.representer.SafeRepresenter$RepresentMap.representData(SafeRepresenter.java:304)
        at org.bukkit.configuration.file.YamlRepresenter$RepresentConfigurationSection.representData(YamlRepresenter.java:23)
        at org.yaml.snakeyaml.representer.BaseRepresenter.representData(BaseRepresenter.java:94)
        at org.yaml.snakeyaml.representer.BaseRepresenter.representMapping(BaseRepresenter.java:156)
        at org.yaml.snakeyaml.representer.SafeRepresenter$RepresentMap.representData(SafeRepresenter.java:304)
        at org.yaml.snakeyaml.representer.BaseRepresenter.representData(BaseRepresenter.java:94)
        at org.yaml.snakeyaml.representer.BaseRepresenter.represent(BaseRepresenter.java:64)
        at org.yaml.snakeyaml.Yaml.dumpAll(Yaml.java:242)
        at org.yaml.snakeyaml.Yaml.dumpAll(Yaml.java:206)
        at org.yaml.snakeyaml.Yaml.dump(Yaml.java:181)
        at org.bukkit.configuration.file.YamlConfiguration.saveToString(YamlConfiguration.java:40)
        at com.nisovin.shopkeepers.ShopkeepersPlugin.saveDataToFile(ShopkeepersPlugin.java:1392)
        at com.nisovin.shopkeepers.ShopkeepersPlugin.access$1200(ShopkeepersPlugin.java:62)
        at com.nisovin.shopkeepers.ShopkeepersPlugin$11.run(ShopkeepersPlugin.java:1302)
        at org.bukkit.craftbukkit.v1_8_R3.scheduler.CraftTask.run(CraftTask.java:71)
        at org.bukkit.craftbukkit.v1_8_R3.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:53)
        ... 3 more