function _getPane(paneId) {
        paneId = _resolvePaneId(paneId);

        if (_panes[paneId]) {
            return _panes[paneId];
        }

        return null;
    }
Retrieves the Pane object for the given paneId
@param {!string} paneId - id of the pane to retrieve
@return {?Pane} the Pane object or null if a pane object doesn't exist for the pane
@private