Praise, and some grave issues

Thank you, Jeroen, for this outstanding plugin!

I really do like Indexy for what makes it unique among free glossary plugins.

What I like best about it is that in addition to what most every glossary plugin does,

1. Indexy allows to have back-links on glossary term pages showing where each glossary term occurs within posts, pages, etc. (An additional shortlink would be great here.)
2. In addition to glossary terms themselves, it allows to define which terms are synonyms of glossary terms, and to treat them in various ways. (Here, also, an additional shortlink showing a list of synonyms of a given term would be great.)

Unfortunately, currently Indexy 1.0.2 does not work well with versions higher than WP 4.3. (I’m running WP-Multisite 4.4.5.) I can basically use it, but not configure it.

• A minor issue is the fact that all terms are being converted to lower case. This may be o.k. in English or Dutch, but is inappropriate in some languages like e.g. German.
• What’s more serious is an issue with Indexy‘s related terms widget.

1. The widget, when added via the Dashboard’s Widgets page, does not show at all.
2. When added via the Customizer, the widget does show initially, as soon as any settings are applied, however, be it that a title is added, that the method of retrieval is changed or that widgets are rearranged, the widget disappears, and it does not reappear, even if the change is reverted.
3. Even when adding a default title within the code itself, no widget title is shown.

• The 2nd major issue is that the plugin settings cannot be saved.

This is what WP_DEBUG has to say:

PHP Warning:  array_keys() expects parameter 1 to be array, string given in /wp-content/plugins/indexy/classes/Indexy_Admin_Options.php on line 351
PHP Warning:  implode(): Invalid arguments passed in /wp-content/plugins/indexy/classes/Indexy_Admin_Options.php on line 351
PHP Warning:  Illegal string offset 'enable' in /wp-content/plugins/indexy/classes/Indexy_Admin_Options.php on line 368
PHP Notice:  Undefined index: method in /wp-content/plugins/indexy/classes/Indexy_Related_Posts_Widget.php on line 80
PHP Notice:  Undefined index: post_limit in /wp-content/plugins/indexy/classes/Indexy_Related_Posts_Widget.php on line 80
PHP Warning:  Indexy::get_related_posts(): invalid value specified for $method in /wp-content/plugins/indexy/classes/Indexy.php on line 399
PHP Notice:  Undefined index: ignore_synonyms in /wp-content/plugins/indexy/classes/Indexy_Glossary_Index_Widget.php on line 63
PHP Notice:  Undefined index: ignore_synonyms in /wp-content/plugins/indexy/classes/Indexy_Glossary_Index_Widget.php on line 206

Using Options Inspector I also noticed that the related post widget’s options appear malformed in that the array contains various sets of conflicting settings, which seem to get saved after all, but incorrectly, which might be why they have no effect.

Perhaps you could find some time to take a look at these issues.
I’d really appreciated it.