TYPO3 Core

The problem can be found here:
https://github.com/TYPO3/TYPO3.CMS/blob/master/typo3/sysext/backend/Classes/Form/FormDataProvider/TcaInline.php#L322-L326

        // For foreign_selector with useCombination $mainChild is the mm record
        // and $combinationChild is the child-child. For 1:n "normal" relations,
        // $mainChild is just the normal child record and $combinationChild is empty.
        $mainChild = $formDataCompiler->compile($formDataCompilerInput);
        if ($parentConfig['foreign_selector'] && $parentConfig['appearance']['useCombination']) {
            try {
                $mainChild['combinationChild'] = $this->compileChildChild($mainChild, $parentConfig);
            } catch (DatabaseRecordException $e) {

The first call to compile() must be caught as well. However generating a flash message there is not the full solution because this will then generate errors in \TYPO3\CMS\Backend\Form\Container\InlineRecordContainer::renderForeignRecordHeader as the required array information is not available.

I can confirm that this error is still present in TYPO3 9 with the following scenario where a model X has a mm-relation to fe_users:

        'users' => array(
            'label' => 'mm relation to fe_users',
            'config' => array(
                'type' => 'inline',
                'foreign_table' => 'fe_users',
                'MM' => 'tx_customext_x_user_mm',
                'MM_opposite_field' => 'uid_foreign',
                'maxitems' => 9999,
                'appearance' => array(
                    'collapseAll' => 1,
                    'levelLinksPosition' => 'none',
                    'showSynchronizationLink' => 1,
                    'showPossibleLocalizationRecords' => 0,
                    'showAllLocalizationLink' => 0,
                ),
            ),

        ),

I traced the cause down to https://github.com/TYPO3/TYPO3.CMS/blob/9.5/typo3/sysext/backend/Classes/Form/FormDataProvider/TcaInline.php#L223

    protected function resolveRelatedRecords(array $result, $fieldName)
    {
        ...

        if ($result['inlineCompileExistingChildren']) {
            foreach ($connectedUidsOfDefaultLanguageRecord as $uid) {
                $compiledChild = $this->compileChild($result, $fieldName, $uid);
                $result['processedTca']['columns'][$fieldName]['children'][] = $compiledChild;
            }
        }
        return $result;
    }

A simple try-catch around the two line with $compiledChild seems to solve this problem. Then the mm record of the deleted user is not been rendered anymore at all.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/67959

Patch set 1 for branch 10.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/68532