Sometimes we get this php error on long page load or load many files are at once

Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 54 bytes)

This is because of memory_limit parameter of php.

If you have sufficient RAM available on server your used you can change this parameter on php.ini

change this - 

memory_limit = 128M

TO

memory_limit = 1024M

usually on ubuntu php.ini is available at "/etc/php/7.x/apache/php.ini" or installation directory on server.

OR 

you can change it using ini_set function of php in your php files

ini_set('memory_limit', '1024M');

 

Note:-

ini_set('memory_limit', '-1'); 

is also a way to sort the problem, but it will give unlimited memory available on server(-1 means).

 

Hope this wiil sort it out your error.