Fix error when toggling 'Include Enchants' checkbox in trader

[teroshan]

• Fix error when toggling 'Include Enchants' in trader

Fixes #6216

Description of the problem being solved:

Lua Error when toggling 'Include Enchants' in trader

Steps taken to verify a working solution:

• Open trader, 'Trade for these items'
• Activate Session Mode
• Use trader to 'Find best' on weapon
• Check 'Include Enchants' checkbox -> ERROR

Link to a build that showcases this PR:

Any build will do

Before screenshot:

See linked issue #6216

After screenshot:

No error