Thanks for reporting, this sounds like a little incompatibility with PHP 7.3+. Anyway, you can turn warnings off in the ACP and let them be logged in a file instead (which should be standard practice).
Warning [2] Illegal string offset 'row_options' - Line: 1643 - File: inc/plugins/bankpipe.php PHP 7.2.24 (Linux)
Warning [2] Illegal string offset 'id' - Line: 1643 - File: inc/plugins/bankpipe.php PHP 7.2.24 (Linux)
The issue extends to bankpipe, with similar errors, though I'm using PHP 7.2.
These errors are shown in a list of 10 warnings when switching to the
Flyover Settings tab. 8 of them are 'row_options' and 'id' being referred to in bankpipe.php, the remaining 2 errors are the ones that the OP reported.
Thanks. Anyway these are just warnings, you can turn them off displaying by enabling error.log logging.