Change your If statement format to:
If IsNull(cmbFormcodeGrouping) Then
Here is what Microsoft says about checking for Null.
Returns a Boolean value that indicates whether an expression contains no valid data (Null).
The required expression argument is a Variant containing a numeric expression or string expression.
IsNull returns True if expression is Null; otherwise, IsNull returns False. If expression consists of more than one variable, Null in any constituent variable causes True to be returned for the entire expression.
The Null value indicates that the Variant contains no valid data. Null is not the same as Empty, which indicates that a variable has not yet been initialized. It is also not the same as a zero-length string (""), which is sometimes referred to as a null string.
Important Use the IsNull function to determine whether an expression contains a Null value. Expressions that you might expect to evaluate to True under some circumstances, such as If Var = Null and If Var <> Null, are always False. This is because any expression containing a Null is itself Null and, therefore, False.