TYPO3 Core

yes please push a change, that would be great!

Okay, I figured out how to replicate the this issue:

Assume we have an plugin with some FlexForm added:

$GLOBALS['TCA']['tt_content']['types']['list']['subtypes_addlist'][$_EXTKEY . '_pi1'] = 'pi_flexform';
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPiFlexFormValue($_EXTKEY . '_pi1', 'FILE:EXT:myextension/Configuration/FlexForms/Test.xml');

This works as expected:

<?xml version="1.0" encoding="utf-8" ?>
<T3DataStructure>
    <meta>
        <langDisable>1</langDisable>
    </meta>
    <sheets>
        <sDEF>
            <ROOT>
                <TCEforms>
                    <sheetTitle>Sheet title</sheetTitle>
                </TCEforms>
                <type>array</type>
                <el>
                    <show_other_field>
                        <TCEforms>
                            <label>Display stuff?</label>
                            <config>
                                <type>check</type>
                            </config>
                        </TCEforms>
                    </show_other_field>
                    <stuff>
                        <TCEforms>
                            <label>Stuff</label>
                            <displayCond>FIELD:show_other_field:=:1</displayCond>
                            <config>
                                <type>check</type>
                            </config>
                        </TCEforms>
                    </stuff>

                </el>
            </ROOT>
        </sDEF>
    </sheets>
</T3DataStructure>

But this does not:

<?xml version="1.0" encoding="utf-8" ?>
<T3DataStructure>
    <meta>
        <langDisable>1</langDisable>
    </meta>
    <sheets>
        <sDEF>
            <ROOT>
                <TCEforms>
                    <sheetTitle>Sheet title</sheetTitle>
                </TCEforms>
                <type>array</type>
                <el>
                    <show_other_field>
                        <TCEforms>
                            <label>Display stuff?</label>
                            <config>
                                <type>check</type>
                            </config>
                        </TCEforms>
                    </show_other_field>
                    <stuff>
                        <TCEforms>
                            <label>Stuff</label>
                            <displayCond>FIELD:show_other_field:=:1</displayCond>
                            <config>

                                <type>select</type>
                                <renderMode>tree</renderMode>
                                <treeConfig>
                                    <parentField>pid</parentField>
                                    <appearance>
                                        <showHeader>TRUE</showHeader>
                                        <width>650</width>
                                    </appearance>

                                </treeConfig>
                                <internal_type>db</internal_type>
                                <size>5</size>
                                <autoSizeMax>20</autoSizeMax>
                                <maxitems>1000</maxitems>
                                <minitems>0</minitems>
                                <foreign_table>sys_file_collection</foreign_table>
                                <foreign_table_where>AND sys_file_collection.hidden = 0 AND (sys_file_collection.sys_language_uid = 0 OR sys_file_collection.l10n_parent = 0)</foreign_table_where>

                            </config>
                        </TCEforms>
                    </stuff>

                </el>
            </ROOT>
        </sDEF>
    </sheets>
</T3DataStructure>

The action FormSelectTreeAjaxController::fetchDataAction is called via ajax and return error 500:
Flex form displayCond on field "stuff" on flex form sheet "sDEF" references a field or field / sheet combination "show_other_field" that might be defined in given data structure but is not found in data values

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/51574

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/51574

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/51574

Patch set 6 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/51574

Patch set 7 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/51574

Patch set 8 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/51574

added a test scenario to styleguide: elements_select -> in flex -> selectTree, select_tree_2