Can't edit page

I am trying to edit a page. The edit box comes up and I can type in it, but when I try and click save I get the following:

Notice Trying to get property ‘MemberGroup’ of non-object ee/EllisLab/ExpressionEngine/Model/Channel/ChannelEntry.php, line 322

Severity: E_NOTICE Notice Trying to get property ‘AssignedChannels’ of non-object ee/EllisLab/ExpressionEngine/Model/Channel/ChannelEntry.php, line 322

Severity: E_NOTICE

Fatal error: Uncaught Error: Call to a member function pluck() on null in /home/thecom41/public_html/ComicsReporter.com/system/ee/EllisLab/ExpressionEngine/Model/Channel/ChannelEntry.php:322 Stack trace: #0 /home/thecom41/public_html/ComicsReporter.com/system/ee/EllisLab/ExpressionEngine/Service/Validation/Validator.php(250): EllisLab\ExpressionEngine\Model\Channel\ChannelEntry->validateAuthorId(‘author_id’, NULL, Array, Object(EllisLab\ExpressionEngine\Service\Validation\Rule\Callback)) #1 [internal function]: EllisLab\ExpressionEngine\Service\Validation\Validator->EllisLab\ExpressionEngine\Service\Validation{closure}(‘author_id’, NULL, Array, Object(EllisLab\ExpressionEngine\Service\Validation\Rule\Callback)) #2 /home/thecom41/public_html/ComicsReporter.com/system/ee/EllisLab/ExpressionEngine/Service/Validation/Rule/Callback.php(30): call_user_func(Object(Closure), ‘author_id’, NULL, Array, Object(EllisLab\ExpressionEngine\Service\Validation\Rule\Callback)) #3 /home/thecom41/public_html/ComicsReporter.com/system/ee/Ell in /home/thecom41/public_html/ComicsReporter.com/system/ee/EllisLab/ExpressionEngine/Model/Channel/ChannelEntry.php on line 322

I am currently using Version: 5.4.2 (I just recently updated, there were other PHP errors when I first logged in, but the upgrade made those errors go away).

Any idea on what this is?

Update: I just updated to the latest 6.0.6 version, hoping it might fix this issue but it didn’t. There error message now looks way different:

Added screen shot due to error code really messed up the formatting on this board.