Big upgrade and a few bits left to sweep up

Hello, I upgraded from 2.9.2 to 4.12 with much help from Derek at Ellislabs (THANKS!) but have a few remaining hitches, that I think are all related to somethings with the channels in the database.

Channel forms can’t find the entries. When using a channel form, I get the following error “The form you submitted contained the following errors You must specify a valid entry to edit.” I do not get this error if I edit from the Control panel.

However, when I go to the CP Entry Manager and search for an entry by a keyword in the entry titles, the entry is not found and I get a host of errors, copied beneath. Any ideas on what to do?

Thank you kindly for helping me button this up!

Notice Trying to get property of non-object ee/EllisLab/ExpressionEngine/Service/EntryListing/EntryListing.php, line 198

Severity: E_NOTICE Notice Trying to get property of non-object ee/EllisLab/ExpressionEngine/Controller/Publish/Edit.php, line 118

Severity: E_NOTICE Notice Trying to get property of non-object ee/EllisLab/ExpressionEngine/Controller/Publish/Edit.php, line 314

Severity: E_NOTICE Warning Cannot modify header information - headers already sent by (output started at ee/legacy/core/Exceptions.php:111) ee/EllisLab/ExpressionEngine/Boot/boot.common.php, line 487

Severity: E_WARNING