There is the following set:

1) A rule template which uses a "$X" variable of type "Integer" in the THEN clause.
2) The $X variable has not been filled out in Template Data. So it is null/empty.
3) The following error is thrown by clicking on "Validate" button:
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Unable to Analyse Expression ...; drools.insert(fact0);: [Error: unable to resolve method using strict-mode: org.drools.spi.KnowledgeHelper.$X_na()][Near:
{...}

] ^ [Line: 2, Column:26]
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As soon as one row in the template data with the X populated with a int/Integer is entered, validation works properly.

If this is the expected behaviour by not populating a variable in template data, so that message should be changed since it is too generic. It should advise that "template data" or "variable X" is not populated.