Check TPI stream is valid before creating it

[sschoener]

The examples erroneously create a TPI stream before checking that it is valid to do so. This is likely a copy-paste error, as on a first look it seems that we are doing the same for DBI. However, for DBI there are two checks, which has probably created this confusion:

[MolecularMatters]

Great, thank you for finding and fixing this!