Describe the bug

In the Gravity Forms form settings page:

1. Click Popup Maker
2. Click Close Popup

Site information

Popup Maker version: Version 1.20.2

Expected behavior

No errors and settings should save.

Current behavior

Fatal error and settings don't save. See above.

Steps to reproduce

See above.

Errors

Fatal error: Uncaught TypeError: Cannot access offset of type string on string in C:\Users\Kimmy\Local Sites\test-2\app\public\wp-content\plugins\popup-maker\includes\integrations\class-pum-gravity-forms.php:272 Stack trace: #0 C:\Users\Kimmy\Local Sites\test-2\app\public\wp-content\plugins\popup-maker\includes\integrations\class-pum-gravity-forms.php(121): PUM_Gravity_Forms_Integation::save() #1 C:\Users\Kimmy\Local Sites\test-2\app\public\wp-includes\class-wp-hook.php(324): PUM_Gravity_Forms_Integation::render_settings_page('') #2 C:\Users\Kimmy\Local Sites\test-2\app\public\wp-includes\class-wp-hook.php(348): WP_Hook->apply_filters('', Array) #3 C:\Users\Kimmy\Local Sites\test-2\app\public\wp-includes\plugin.php(517): WP_Hook->do_action(Array) #4 C:\Users\Kimmy\Local Sites\test-2\app\public\wp-content\plugins\gravityforms\form_settings.php(64): do_action('gform_form_sett...') #5 C:\Users\Kimmy\Local Sites\test-2\app\public\wp-content\plugins\gravityforms\gravityforms.php(3953): GFFormSettings::form_settings_page('2') #6 C:\Users\Kimmy\Local Sites\test-2\app\public\wp-includes\class-wp-hook.php(324): GFForms::forms('') #7 C:\Users\Kimmy\Local Sites\test-2\app\public\wp-includes\class-wp-hook.php(348): WP_Hook->apply_filters('', Array) #8 C:\Users\Kimmy\Local Sites\test-2\app\public\wp-includes\plugin.php(517): WP_Hook->do_action(Array) #9 C:\Users\Kimmy\Local Sites\test-2\app\public\wp-admin\admin.php(259): do_action('toplevel_page_g...') #10 {main} thrown in C:\Users\Kimmy\Local Sites\test-2\app\public\wp-content\plugins\popup-maker\includes\integrations\class-pum-gravity-forms.php on line 272
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Additional context

$settings = sanitize_key( wp_unslash( $_POST['gforms-pum'] ) );

This returns an empty string which causes the fatal errors for the Close Popup and Open Popup Gravity Forms form settings.