pfSense

There must be something unique to your configuration triggering this. It does not appear to be a general issue affecting all users.

Please attach the <dnshaper></dnshaper> portion of the config.xml file from the affected firewall.

I looked at the code - I guess that $sform is null.

By default $dontshow is false. So by default it will expect $sform to not be null.
In case "add", $sform is not set in all cases and $dontshow is not set to true when $sform is not set. Same in "show", "enable" and "disable". $input_errors is set though.
When "$action" is "add", the code is supposedly not entered but << $action != "add" >> is strictly speaking not the correct comparison, it should be
<< $action !== "add" >> (0!="add" is false!). This is however not the cause.

This error may have occurred while deleting queues.

The system log shows this:
./system.log:Feb 8 20:18:53 pf3 php-fpm²⁷¹⁶⁵: /firewall_shaper_vinterface.php: PHP ERROR: Type: 1, File: /usr/local/www/firewall_shaper_vinterface.php, Line: 415, Message: Uncaught Error: Call to a member function addGlobal() on null in /usr/local/www/firewall_shaper_vinterface.php:415 Stack trace: #0 {main} thrown
./system.log:Feb 8 20:18:53 pf3 php-fpm²⁷¹⁶⁵: /firewall_shaper_vinterface.php: New alert found: PHP ERROR: Type: 1, File: /usr/local/www/firewall_shaper_vinterface.php, Line: 415, Message: Uncaught Error: Call to a member function addGlobal() on null in /usr/local/www/firewall_shaper_vinterface.php:415 Stack trace: #0 {main} thrown

At that timestamp, the nginx.log does show a "show" action from a "delete" action:
nginx.log:Feb 8 20:18:44 pf3 pf3.domain.name nginx: 192.168.5.20 - - [08/Feb/2019:20:18:44 +0100] "GET /firewall_shaper_vinterface.php HTTP/2.0" 499 0 "https://192.168.5.251/firewall_shaper_vinterface.php?pipe=outMailOthers&queue=outMailOthers&action=delete" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/72.0.3626.96 Safari/537.36"
nginx.log:Feb 8 20:18:53 pf3 pf3.domain.name nginx: 192.168.5.20 - - [08/Feb/2019:20:18:53 +0100] "GET /firewall_shaper_vinterface.php?pipe=outMailOthers&queue=outMailOthers&action=show HTTP/2.0" 200 6011 "https://192.168.5.251/firewall_shaper_vinterface.php?pipe=outMailOthers&queue=outMailOthers&action=delete" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like

It looks like the "outMailOthers" was deleted and then requested to be shown. So in << case "show", the $sform would not have been set.
n
$dfltmsg is false, $dontshow is false, $action is "show", so th enext code is executed with an $sform that is unset.

$sform->addGlobal(new Form_Button(
                                'delete',
                                ($queue && ($qname != $pipe)) ? 'Delete this queue':'Delete Limiter',
                                $url,
                                'fa-trash'
                        ))->addClass('btn-danger');

It is not clear how I got from the delete action to the show action. Possibly a deleted queue was still shown - I may have opened multiple pages to delete queues and I may have clicked on a link to the deleted queue.

IMHO, nothing to do with my configuration, and some code here that must be corrected and be made more robust:
- Compare strings with "===";
- Improve coping with $sform not being set and $input_errors.

As requested, here is the current dnshaper section:
<dnshaper>
<queue>
<name>in</name>
<number>1</number>
<qlimit></qlimit>
<plr></plr>
<description><![CDATA[In]]></description>
<bandwidth>
<item>
<bw>2500</bw>
<burst></burst>
<bwscale>Kb</bwscale>
<bwsched>none</bwsched>
</item>
</bandwidth>
<enabled></enabled>
<buckets></buckets>
<mask>none</mask>
<maskbits></maskbits>
<maskbitsv6></maskbitsv6>
<delay>0</delay>
</queue>
<queue>
<name>out</name>
<number>2</number>
<qlimit></qlimit>
<plr></plr>
<description><![CDATA[out]]></description>
<bandwidth>
<item>
<bw>500</bw>
<burst></burst>
<bwscale>Kb</bwscale>
<bwsched>none</bwsched>
</item>
</bandwidth>
<enabled></enabled>
<buckets></buckets>
<mask>none</mask>
<maskbits></maskbits>
<maskbitsv6></maskbitsv6>
<delay>0</delay>
</queue>
</dnshaper>

Extra note:

The next line shows that I did get from the queue "delete" action to the "show" action of the same queue.

nginx.log:Feb 8 20:18:53 pf3 pf3.domain.name nginx: 192.168.5.20 - - [08/Feb/2019:20:18:53 +0100] "GET /firewall_shaper_vinterface.php?pipe=outMailOthers&queue=outMailOthers&action=show HTTP/2.0" 200 6011 "https://192.168.5.251/firewall_shaper_vinterface.php?pipe=outMailOthers&queue=outMailOthers&action=delete"

It looks like the list of queues to show is computed before the queue is deleted during the delete action?