Compare table PHP Warnings

[dwanjuki]

Describe the bug
PHP warnings (Undefined array key) are triggered when using the Advanced Levels Page compare_table layout if improperly formatted content is provided in the compare attribute.
https://github.com/strangerstudios/pmpro-advanced-levels-shortcode/blob/1.2/templates/levels-compare_table.php#L231-L236

To Reproduce
Steps to reproduce the behavior:

1. Add the advanced levels shortcode to a page with invalid compare attribute content for the compare_table layout, e.g. [pmpro_advanced_levels levels="1,2" layout="compare_table" compare="hello"]
2. Visit the advanced levels page
3. See warnings/Check debug logs

Expected behavior
No PHP warnings.

Isolating the problem (mark completed items with an [x]):

• I have deactivated other plugins and confirmed this bug occurs when only Paid Memberships Pro plugin is active.
• This bug happens with a default WordPress theme active, or Memberlite.
• I can reproduce this bug consistently using the steps above.

WordPress Environment

Details

Advanced Levels Shortcode 1.2