Properties like TMP_Text.fontMaterial cannot be accessed if the object reference type is TextMeshPro even though it inherits from TMP_Text. You end up having to cast back to the TMP_Text type to use the property:

var fontRenderer1 = renderer.GetComponent<TextMeshPro>();

f(fontRenderer1 != null) fontRenderer1.fontMaterial = ...; // Throws not exposed in Udon error.

var fontRenderer2 = renderer.GetComponent<TMP_Text>(); // Can throw an error if there is no TextMeshPro component on the object

if(fontRenderer2 != null) fontRenderer2.fontMaterial = ...; // Accessible in Udon

var fontRenderer3 = (TMP_Text)renderer.GetComponent<TextMeshPro>();

f(fontRenderer3 != null) fontRenderer3.fontMaterial = ...; // Accessible in Udon