How to correct type errors with mypy library

I check my code using mypy. I got these errors:

1. I have an object names: List[str], for len(names) I get

   Argument 1 to "len" has incompatible type "Optional[List[str]]"; expected "Sized"

   When I try to index like names[i] I get:

   Value of type "Optional[List[str]]" is not indexable

2. I have an object matrix: List[List[int]], similarly matrix[i][j]

   Value of type "Optional[List[List[int]]]" is not indexable

3. I have an object

    g = {"1": ["2"],
     "2": ["3"]}

   annotated as Dict[str, List[str]], I get an error:

   Incompatible return value type (got "Dict[str, object]", expected "Dict[str, List[str]]")

4. I don't understand, why I got this type. If I change it to Dict[str, object], I get another errors in my code.

5. When I try to use my object:

    for obj in g:
        g.get(obj)

   I get this error:

   Argument 1 to "extend" of "list" has incompatible type "Optional[List[str]]"; expected "Iterable[str]"

6. I am new in python and don't understand principles of working with mypy- how should I annotate types so as not to get similar errors?