Error when Paid Memberships Pro is disabled.

[andrewlimaza]

Describe the bug If Paid Memberships Pro core is disabled, fatal error to undefined function.

To Reproduce

1. Keep Approvals Add On enabled.
2. Disable Paid Memberships Pro core.

Expected behavior We should check to see if Paid Memberships Pro core is active.

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

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