cannot open the activity app after editing/opening a mindmap

I am currently on v20, NC 18.0.2, Ubuntu 18.04, PHP 7.2

After opening/editing a mindmap I cannot open the activity app with this user.

To reproduce:

0. install mindmap v20
1. open/edit a mindmap
2. close and save
3. try to open the activity app
4. See a never ending spinning wheel

expected:

I shouldn't see this spinning wheel but instead the activity stream.

These errors are in the logs:

1.

[PHP] Error: copy(/var/www/nextcloud/core/img/filetypes/mindmap.svg): failed to open stream: Permission denied at /var/www/nextcloud/apps/files_mindmap/lib/Migration/InstallStep.php#41

POST /settings/apps/enable
from 87.123.148.145 by admin at 2020-03-17T21:26:25+01:00

2.

[PHP] Error: file_put_contents(/var/www/nextcloud/core/js/mimetypelist.js): failed to open stream: Permission denied at /var/www/nextcloud/core/Command/Maintenance/Mimetype/UpdateJS.php#57

POST /settings/apps/enable
from 87.123.148.145 by admin at 2020-03-17T21:26:25+01:00

3.

[no app in context] Error: Exception: substr() expects parameter 3 to be integer, boolean given at <>

0. /var/www/nextcloud/lib/private/AppFramework/App.php line 125
   OC\AppFramework\Http\Dispatcher->dispatch(OCA\Activity\Controller\APIv2 {}, "getFilter")
1. /var/www/nextcloud/lib/private/AppFramework/Routing/RouteActionHandler.php line 47
   OC\AppFramework\App::main("APIv2Controller", "getFilter", OC\AppFramework\ ... {}, {filter: "all",_ ... "})
2. <>
   OC\AppFramework\Routing\RouteActionHandler->_invoke({filter: "all", ... "})
3. /var/www/nextcloud/lib/private/Route/Router.php line 299
   call_user_func(OC\AppFramework\ ... {}, {filter: "all",_ ... "})
4. /var/www/nextcloud/ocs/v1.php line 82
   OC\Route\Router->match("/ocsapp/apps/ac ... l")
5. /var/www/nextcloud/ocs/v2.php line 24
   require_once("/var/www/nextcloud/ocs/v1.php")

GET /ocs/v2.php/apps/activity/api/v2/activity/all?format=json&previews=true&since=0
from 87.123.148.145 by admin at 2020-03-17T21:28:06+01:00

Reproduce the mindmap edit/save flow, then read apps/files_mindmap/lib/Migration/InstallStep.php around line 41 and trace the activity API request shown in the logs. Check the referenced permission errors and substr() failure; done means the activity stream opens after editing a mindmap without the reported errors.