WordPress php error

I am upgraded to 3.9 when it was issued

I got this warning today when working with some plugins – first time since I upgraded.

Warning: Cannot modify header information – headers already sent by (output started at /home4/mia/public_html/wp-content/themes/customizr-child/functions.php:1) in /home4/mia/public_html/wp-content/plugins/restricted-to-adults/rta.php on line 29

After I read the various threads on the support forum regarding this I disabled this RTA plugin and one other to see what happened if I reactivated the other plugin and this is what I get

Warning: Cannot modify header information – headers already sent by (output started at /home4/mia/public_html/wp-content/themes/customizr-child/functions.php:1) in /home4/mia/public_html/wp-includes/pluggable.php on line 1121

Warning: Cannot modify header information – headers already sent by (output started at /home4/mia/public_html/wp-content/themes/customizr-child/functions.php:1) in /home4/mia/public_html/wp-includes/pluggable.php on line 1121

It seems that random errors are coming out depending on what plugin you activate/deactivate.

I’ve checked the wp-config.php file and the header is as it should be <?php with nothing else there.

Any ideas?