Heading cannot have dot in the name

[wernerkrauss]

When setting exportfields like

['Birthdate' => 'Geb.Datum']

it fails with:

[Emergency] Uncaught LogicException: Geb\ is not a relation/field on MyDataObject

Trace:

SilverStripe\ORM\DataObject->relObject(Geb\)
DataObject.php:3111
SilverStripe\ORM\DataObject->relField(Datum)
GridField.php:842
SilverStripe\Forms\GridField\GridField->getDataFieldValue(MyDataObject, Geb\.Datum)
GridFieldExcelExportButton.php:133
Level51\ExcelExport\GridFieldExcelExportButton->handleExcelExport(SilverStripe\Forms\GridField\GridField)
GridFieldExcelExportButton.php:57