The method getShieldState() does not exist on Stu\Lib\Map\VisualPanel\...\Data\CellDataInterface. It seems like you code against a sub-type of Stu\Lib\Map\VisualPanel\...\Data\CellDataInterface such as Stu\Lib\Map\VisualPanel\...r\Data\ColonyShieldData.
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Ignorable by Annotation
)
If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation
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if (!$data->/** @scrutinizer ignore-call */ getShieldState()) {