[Bug] error during formula creation

[TimonPllkrn]

Describe the bug
The creation of mathematical formulas often leads to errors in the creation of the video.

To Reproduce
Steps to reproduce the behavior:

1. Got to Infoprovider erstellen
2. Add api data (e.g. http://api.open-notify.org/iss-now.json?callback=?)
3. Select number at Dateiauswahl (e.g. timestamp)
4. skip Arrayverarbeitung
5. select a number value and multiply by 8 (e.g. timestamp * 8)
6. Finish video creation
7. execute video job
8. Error: TransformError: On Type 'calculate', TypeError: expected str, got NoneType

Traceback

traceback: Traceback (most recent call last): File "/home/appuser/visuanalytics/analytics/util/step_errors.py", line 193, in new_func return func(values, *args, **kwargs) File "/home/appuser/visuanalytics/analytics/transform/transform.py", line 88, in calculate action_func(values, data) File "/home/appuser/visuanalytics/analytics/transform/calculate.py", line 225, in calculate_multiply _bi_calculate(values, data, operator.mul) File "/home/appuser/visuanalytics/analytics/transform/calculate.py", line 204, in _bi_calculate left = data.get_data(value_left, values, numbers.Number) File "/home/appuser/visuanalytics/analytics/control/procedures/step_data.py", line 166, in get_data key = self.__formatter.format(key, data) File "/usr/lib/python3.6/string.py", line 190, in format return self.vformat(format_string, args, kwargs) File "/usr/lib/python3.6/string.py", line 194, in vformat result, _ = self._vformat(format_string, args, kwargs, used_args, 2) File "/usr/lib/python3.6/string.py", line 204, in _vformat self.parse(format_string): File "/usr/lib/python3.6/string.py", line 288, in parse return _string.formatter_parser(format_string) TypeError: expected str, got NoneType The above exception was the direct cause of the following exception: Traceback (most recent call last): File "/home/appuser/visuanalytics/analytics/control/procedures/pipeline.py", line 258, in start self.__steps[self.__current_step].get("call", lambda: None)(self.__config, data) File "/home/appuser/visuanalytics/analytics/util/step_errors.py", line 193, in new_func return func(values, *args, **kwargs) File "/home/appuser/visuanalytics/analytics/transform/transform.py", line 39, in transform trans_func(transformation, data) File "/home/appuser/visuanalytics/analytics/util/type_utils.py", line 27, in type_func return func(values, data, *args, **kwargs) File "/home/appuser/visuanalytics/analytics/util/step_errors.py", line 198, in new_func raise error(values) from e visuanalytics.analytics.util.step_errors.TransformError: On Type 'calculate', TypeError: expected str, got NoneType

Expected behavior
All entries in the formula editor should be valid calculations and thus should not lead to errors in the creation.