TYPO3 Core

I get the same error message in 6.2.3 but I think it should be fixed by update WorkspaceSelectorToolbarItem.php
just replace this line:
$availableWorkspaces = \TYPO3\CMS\Workspaces\Service\WorkspaceService::getAvailableWorkspaces();

with the following lines (from the same file some lines above):

/** @var \TYPO3\CMS\Workspaces\Service\WorkspaceService $wsService */
$wsService = GeneralUtility::makeInstance('TYPO3\\CMS\\Workspaces\\Service\\WorkspaceService');
$availableWorkspaces = $wsService->getAvailableWorkspaces();