An error occurs when importing items using 'mm i import'

Summary​​ Error when using mm i import, unable to import items into MythicMobs.

​​Steps to reproduce​​

Hold an item in your hand. Execute the command /mm i import test. ​​Current behavior​​ The item is not saved to MythicMobs.

​​Intended correct behavior​​ The item should be successfully imported into MythicMobs.

​​Server log file​​ https://mclo.gs/3oYswEG

​​Debug log snippet​​ The relevant error can be seen in the log file provided above: 'Cannot invoke "org.bukkit.Material.getMaxStackSize()" because "this.material" is null'.

​​Proposed fixes​​ Fix the reported error: 'Cannot invoke "org.bukkit.Material.getMaxStackSize()" because "this.material" is null'.