KeyError in get_lines_string

The trp code called by `get_lines_string()` expects the `cid` key to be in `blockMap`. If it's not, an exception is thrown in trp ___int____.py:

```
line 153, in __init__
if (blockMap[cid]["BlockType"] == "WORD"):
~~~~~~~~^^^^^
KeyError: 'd019e793-f973-4326-87ef-a4bfa06619ab'
```

Start with get_lines_string() and the referenced trp ___int____.py at line 153, where blockMap[cid] raises the KeyError. Trace how cid and blockMap are built, then determine the intended behavior when the key is absent; completion should prevent the reported exception and include a test for this case.

Written by the indexing model from the issue text.