TYPO3 Core

yes it is related to #71436 as stated above

Tested this issue with the latest 7.6.1-dev and this special case is not resolved

Well - in the case of gridelements the stuff that is breaking has nothing to do with the hook but fails due to the fact that there is now "parent" in \TYPO3\CMS\Backend\Controller\FormInlineAjaxController::createAction..

// Parent, this table embeds the child table
        $parent = $inlineStackProcessor->getStructureLevel(-1);
        $parentFieldName = $parent['field'];

        if (MathUtility::canBeInterpretedAsInteger($parent['uid'])) {
            $command = 'edit';
            $vanillaUid = (int)$parent['uid'];
            $databaseRow = [
                // TcaInlineExpandCollapseState needs the record uid
                'uid' => (int)$parent['uid'],
            ];
        } else {
            $command = 'new';
            $databaseRow = [];
            $vanillaUid = (int)$inlineFirstPid;
        }
        $formDataCompilerInputForParent = [
            'vanillaUid' => $vanillaUid,
            'command' => $command,
            'tableName' => $parent['table'],
            'databaseRow' => $databaseRow,
            'inlineFirstPid' => $inlineFirstPid,
            'columnsToProcess' => [
                $parentFieldName
            ],
            // Do not resolve existing children, we don't need them now
            'inlineResolveExistingChildren' => false,
        ];

Without a parent there is no table thus giving you:

Uncaught TYPO3 Exception
#1437654409: No $tableName given (More information)

InvalidArgumentException thrown in file
/var/www/typo3-7x.io/typo3/sysext/backend/Classes/Form/FormDataCompiler.php in line 81.

AFAICS there is no hook or gridelements specific userfunc involved until that point, but there is a return call_user_func_array($target, array($request, $response)); within the RouteDispatcher

OK - with latest TYPO3 master it as changed to

Uncaught TYPO3 Exception
#1: PHP Catchable Fatal Error: Argument 1 passed to TYPO3\CMS\Backend\Controller\FormInlineAjaxController::addFlexFormDataStructurePointersFromAjaxContext() must be of the type array, null given, called in /var/www/typo3-7x.io/typo3/sysext/backend/Classes/Controller/FormInlineAjaxController.php on line 74 and defined in /var/www/typo3-7x.io/typo3/sysext/backend/Classes/Controller/FormInlineAjaxController.php line 873 (More information)

Will dig deeper now. :-)

Any news on that bug?
It's still not fixed. There are a few other related tickets but none of them has solved the problem yet.
There is still no way to add images using 7.6.2, fluid_styled_content & gridelements.

I found the solution for the bug (at least the solution for me. It seems that I had another problem that resolved in the same error).
I had to activate the mb_string extension in php and the problem was gone. I immediately could add images without any errors.

I had the same issue, and it turned out to be an issue in Flux.
It's already solved in commit https://github.com/FluidTYPO3/flux/commit/6508a5fb26af1511ab5d6309d7e6ffffcc222f1d
however it's not released yet to TER (you need to checkout "development" branch of flux)