Connect to DynamoDB Local dialog validation uses list selection all the time

If you uncheck "Start new DynamoDB Local process", the list selection is still used in validation.
If a list item is not selected (shown in image below), the error "You must select a version of the DynamoDB Local to start." is shown.


Fix the dialog validation to not use the list selection when the checkbox is unchecked.

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Workaround:
* Check "Start new DynamoDB Local process"
* Install one of the versions if none are installed (this downloads a jar file)
* Select one of the installed version rows
* Uncheck "Start new DynamoDB Local process"
Now when you click Ok, it will use your existing DynamoDB Local host

Related: #47

Start by locating the “Connect to DynamoDB Local” dialog and its validation logic, then reproduce the case where “Start new DynamoDB Local process” is unchecked and no list row is selected. Done means that validation accepts the unchecked state while still requiring a selected version when the checkbox is checked; verify both paths in the dialog.

Written by the indexing model from the issue text.