Why do I receive a Mutator error when modifying an Xml value in Sql

Question

I have the following stored procedure:

ALTER PROCEDURE [dbo].[UpPro]
(
    @PN varchar(200),
    @xml xml,
    @gVa varchar(10),
)
AS
/* update the gender */
BEGIN
    SET NOCOUNT ON;

    Select @gVa = t1.[Gender] 
    From [myDb].[dbo].[myTable1] t1 --replace Value2 and table to the table which is updated through SSIS
    Where t1.Name = @PN

    PRINT @gVa //displays 'F'

    Set @xml.modify('replace value of (/root/Phys/gender/text())[1] with sql:variable("@gVa")');
END

/* once all the node has been "temporarily" changed, update the table columns for the provider */
BEGIN
    --update the table after casting dummy xml variable
    Update [myDb].[dbo].[TC]
    Set [chtml] = cast(cast(@xml as nvarchar(max)) as ntext)
    Where [ctitle] = @PN
END

When I run the query, I get the following error:

Msg 5302, Level 16, State 1, Procedure UpPro, Line 115
Mutator 'modify()' on '@xml' cannot be called on a null value.

How can I resolve the error. I am trying to update xml value in a column (chtml) which is of type ntext in the TC table.

Please let me know if I have to provide more information.

Just to test out the code, I just tried the following and it still gave the same error:

DECLARE @gVa varchar(10)
DECLARE @xml xml

Select @gVa = t1.[Gender] 
From [myDb].[dbo].[myTable1] t1 --replace Value2 and table to the table which is updated through SSIS
Where t1.Name = 'Doctor 1'
PRINT @gVa

If @xml IS NOT NULL
BEGIN
    Set @xml.modify('replace value of (/root/Phys/gender/text())[1] with sql:variable("@gVa")');
END
Else
BEGIN
    PRINT 'NOT WORK'
END

Keeps printing NOT WORK

Original column (chtml) data:

<?xml version="1.0" encoding="UTF-8"?>
<root>
    <Phys>
        <gender>M</gender>
    </Phys>
</root>

After the above SP executes, the gender should be F and not M

Answer 1

The error actually says it all, @XML is null and that is not allowed.

Repro:

declare @X xml;

set @X.modify('replace value of text()[1] with "1"');

Msg 5302, Level 16, State 1, Line 4 Mutator 'modify()' on '@X' cannot be called on a null value.

Check for null before you modfy.

declare @X xml;

if @X is not null
  set @X.modify('replace value of text()[1] with "1"');