Selects the option whose export value, else displayed text, is value (the text itself in an editable combo
box when no option has it), or nothing with null, and regenerates the widgets' appearances.
public void SetValue(string? value)
An option's export value or displayed text; any text for an editable combo box; or null.
ISO 32000-2 §12.7.5.4 and Table 234, as SetSelectedIndices(IEnumerable<Int32>) describes.