A valid file order source should not be considered invalid

Current Situation

Valid file order sources are considered invalid in some situations.
Example how to reproduce

• Create Workflow W1
• Do not define the workflow variable "file"
• Create a File Order Source F1
• set all required values
• for the directory to be watched use e.g. c:\temp
• F1 now is invalid as W1 does not define the "file" variable
• Open W1
• Define the "file" variable
• Open F1

F1 now still is invalid.
The message that is displayed is {{}}

$.directoryExpr: Error in expression: Parsing failed at position 2 “c❓:\temp” · Expected character '('}

• Change the directory to X
• F1 now becomes valid
• Switch to W1
• Remove the variable "file"
• Switch to F1
• F1 is still valid but should be invalid as the file variable is not set in W1

Desired Behavior

A File Order Source should be displayed as invalid only when it is invalid.
Changing the assigned workflow by removing the "file" variable should change the status of the File Order Source to invalid.

Maintainer Note

When saving a workflow, it will be checked whether the workflow is referenced in a file order source and the required "file" variable will be added if necessary.