Mypy typing error

[WannaBePythonDev]

when posting errors, please, always post the entire, unaltered error traceback as it contains valuable diagnostic information.

Error:
{
	"resource": "~/Intro to Logic/Projects/propositions/syntax.py",
	"owner": "python",
	"code": "error",
	"severity": 8,
	"message": "Argument 1 to \"formula_obj_to_string\" of \"Formula\" has incompatible type \"Optional[Formula]\"; expected \"Formula\"",
	"source": "mypy",
	"startLineNumber": 134,
	"startColumn": 61,
	"endLineNumber": 134,
	"endColumn": 61
}

{
	"resource": "~/Intro to Logic/Projects/propositions/syntax.py",
	"owner": "python",
	"code": "error",
	"severity": 8,
	"message": "Argument 2 to \"create_binary_formula\" of \"Formula\" has incompatible type \"Optional[Formula]\"; expected \"Formula\"",
	"source": "mypy",
	"startLineNumber": 175,
	"startColumn": 31,
	"endLineNumber": 175,
	"endColumn": 31
}

{
	"resource": "~/Intro to Logic/Projects/propositions/syntax.py",
	"owner": "python",
	"code": "error",
	"severity": 8,
	"message": "Argument 3 to \"create_binary_formula\" of \"Formula\" has incompatible type \"Optional[Formula]\"; expected \"Formula\"",
	"source": "mypy",
	"startLineNumber": 175,
	"startColumn": 46,
	"endLineNumber": 175,
	"endColumn": 46
}

    @staticmethod
    def negate_formula(formula: Formula) -> str:
        """Negates a formula.

        Parameters:
            formula: A formula to negate.

        Returns:
            A string reresenting the formula negated.
        """
        return formula.root + Formula.formula_obj_to_string(formula.first)

    @staticmethod
    def create_binary_formula(
        binary_operator: str, first_sub_formula: Formula, second_sub_formula: Formula
    ) -> str:
        """Assmebles a binary formula out of two sub-formulas.

        Parameters:
            binary_operator: The binary operator applied on the two sub-formulas.
            first_sub_formula: The first sub-formula.
            second_sub_formula: The second sub-formula.

        Returns:
            A string reresenting the suitable binary formula.
        """
        return (
            OPEN_BINARY_FORM
            + Formula.formula_obj_to_string(first_sub_formula)
            + binary_operator
            + Formula.formula_obj_to_string(second_sub_formula)
            + CLOSE_BINARY_FORM
        )

This is all the related stuff.