[FIX] barcodes: tests: don't add undefined widget to registry
[FIX] barcodes: tests: don't add undefined widget to registry

Resetting the key 'test_barcode_handler' to its value before the
test was technically the best solution, as a test should leave the
environment unchanged. However, in this case, the widget is
undefined (there is obviously no 'test_barcode_handler').
Whereas this should not be a problem (the fields in the registry
are supposed to be accessed using get(), and the presence of a
key mapping undefined is equivalent to the absence of the key in
this case), this breaks a test in Studio if the latter one is
executed after the barcode test. This is because Studio browses the
map of the field registry to access each field's prototype. When it
encounters an undefined value, it crashes.

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