[Plugin: WP-Footnotes] Undefined index: symbol

In fact, the plugin needs a few improvements as default values for arrays, compact these arrays…
but, in this case and in a quickly way, line 200 change by this:

if ( !empty($identifiers[$i]['text']) )
	$footnotes[$identifiers[$i]['use_footnote']]['text']	= $identifiers[$i]['text'];
if ( !empty($identifiers[$i]['symbol']) )
	$footnotes[$identifiers[$i]['use_footnote']]['symbol']	= $identifiers[$i]['symbol'];

??

And… a new small code improvement (right after):
Change this redundant and innecessary code:

// Footnotes and identifiers are stored in the array
$use_full_link		= false;
if ( is_feed() ) $use_full_link	= true;
if ( is_preview() ) $use_full_link = false;

$use_full_link is set to false except if is_feed(), we dont’t need set to false, again, if is_preview().
So, set allways to false and true if is_feed():

// Footnotes and identifiers are stored in the array
$use_full_link		= (bool) is_feed() ? true : false;

or more strict:

// Footnotes and identifiers are stored in the array
$use_full_link		= (bool) !is_feed() ? false : true;

More improvements, 2 internal – private helpers _getLinkFormat() for links and _getCurrentOption():
https://pastebin.com/TVNgrM7W

More (perfomance): We don’t need call extra functions in each instance of loop for static values as permalink…

Near line 208:
[Code moderated as per the Forum Rules. Please use the pastebin]