Sequence Editor should warn about a missing command dictionary

[cartermak]

Currently, the sequence editor will let you start writing a sequence or load a JSON without having selected a command dictionary from the dropdown menu. This leads to cryptic error messages in the editor without context (e.g., below) instead of pointing the user to the missing dictionary.

While the description field does specify that the dictionary is required, it would be helpful if the UI either highlighted that a dictionary was missing or prevented editing/uploading a JSON until a dictionary is selected.

[cartermak]

Probably can be replaced by #1378

[cartermak]

Closing as OBE since we now have parcels and #1378