Properly check for collapsed notes option.

src/Plugin/Filter/ReferenceFootnotesFilter.php

@@ -129,7 +129,7 @@ class ReferenceFootnotesFilter extends FilterBase {
     if ($op == 'prepare') {
       // In the 'prepare' case, the first argument contains the options to use.
       // The name 'matches' is incorrect, we just use the variable anyway.
-      $opt_collapse = $matches;
+      $opt_collapse = isset($matches['footnotes_collapse']) && $matches['footnotes_collapse'];
       return 0;
     }