As found in MDL-40971 Clean theme throws an exception if $PAGE->set_pagelayout() is changed after output is initialised.

Obviously in each particular case we should fix the call to set_pagelayout() like Mary did in MDL-40971.

But in the generic case since we don't show any warnings (they are commented out) inside the set_pagelayout() function, theme should be more tolerant to it as well. Standard-like themes have some php code inside layout files checking if block regions are not empty:
https://github.com/marinaglancy/moodle/blob/master/theme/base/layout/general.php#L6

Bootstrapbase/clean should do something similar. AFAIK samhemelryk was trying to avoid php code in layout files, maybe we can insert the check in renderer then?

diff --git a/lib/outputrenderers.php b/lib/outputrenderers.php

index 56b6f56..5a0bf27 100644

--- a/lib/outputrenderers.php

+++ b/lib/outputrenderers.php

@@ -3199,7 +3199,12 @@ EOD;

             'data-blockregion' => $displayregion,

             'data-droptarget' => '1'

         );

-        return html_writer::tag($tag, $this->blocks_for_region($region), $attributes);

+        if ($this->page->blocks->region_has_content($region, $this)) {

+            $content = $this->blocks_for_region($region);

+        } else {

+            $content = '';

+        }

+        return html_writer::tag($tag, $content, $attributes);

     }

     /**