Bugfix with terminating all digital input tasks

Description

Small bugfix

Everytime you stopped the new time series streamer while having multiple input channels configured, you got an exception. This PR fixes it.

IQO Setup 9

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

[x] My code follows the code style of this project.
[ ] I have documented my changes in the changelog (documentation/changelog.md)
[ ] My change requires a change to the documentation.
[ ] I have updated the documentation accordingly.
[ ] I have added/updated for the module the config example in the docstring of the class accordingly.
[x] I have checked that the change does not contain obvious errors (syntax, indentation, mutable default values).
[ ] I have tested my changes using 'Load all modules' on the default dummy configuration with my changes included.
[ ] All changed Jupyter notebooks have been stripped of their output cells.