The paragraph object in the Word has a property called Range. Within this Range object has a property called Cells.
For paragraph that are not in a table, this prop
You can't call the Cells method unless the paragraph is in a table. You need to use a different method to determine whether the range is in a table.
You can use either...
paragraph.Range.Tables.Count > 0
...or...
paragraph.Range.Information(wdWithinTable)
Note that the second one looks more obvious, but is actually slower (only a problem if you're doing this inside a loop).