TYPO3 Core

Source: http://www.jambage.com/kontakt/forum/shop-system/backend/list/6967.html

Same for line 539 in InlineControlContainer.php

            foreach ($possibleRecords as $p) {
                if (!in_array($p[1], $uniqueIds)) {
                    $opt[] = '<option value="' . htmlspecialchars($p[1]) . '">' . htmlspecialchars($p[0]) . '</option>';
                }
            }

changed to:

        if (!empty($possibleRecords)) {
            foreach ($possibleRecords as $p) {
                if (!in_array($p[1], $uniqueIds)) {
                    $opt[] = '<option value="' . htmlspecialchars($p[1]) . '">' . htmlspecialchars($p[0]) . '</option>';
                }
            }
        }

The author arguments, that this is not a bug in tt_products, but an incompatibility introduced with TYPO3 7.6, connected with php errors in TYPO3 7.6

Workaround:
In the file tca.php the lines have to be added:

'type' => 'select',
'renderType' => 'selectSingle',

After each "'type' => 'select',"
there has to be a line "'renderType' => 'selectSingle',"
in case there is no render type.

Also the file ext_tables.php must be modified.
The document
https://docs.typo3.org/typo3cms/extensions/core/latest/Changelog/7.6/Deprecation-69822-DeprecateSelectFieldTca.html
describes it.

(Source: http://www.jambage.com/kontakt/forum/shop-system/backend/list/6967.html)

Hey.

Setting TCA from / within ext_tables was discouraged with 6.2 already and migration of non-clean TCA for core version 7 only if ext:compatibility6 is loaded, which in turn results in a significant performance loss!

So, this is still an issue with tt_product and not in core. The extension should finally put its TCA definition out of ext_tables.php and to Configuration/TCA or Configuration/TCA/Overrides AND set the renderType for type=select TCA fields. The core PHP warning will vanish, then. This was mentioned in https://docs.typo3.org/typo3cms/extensions/core/latest/Changelog/7.3/Deprecation-67229-TcaChanges.html#impact together with an explanation of other TCA changes.