Initialization assignment should not allow variable to be initialized on right-hand side of the expression

[damascenodiego]

Currently initialized variables can be used (without warnings) on the right-hand side of the expression.
This behavior has led to cryptic error messages indicating that a variable is set as None.

A warning/error message should be reported to the user.

[Yuri-Blankenstein-TNO]

@damascenodiego, could you add an example data expression please?