Incorrect error message when setting end date on parent platform

Summary

When changing end date of parent platform from "open end" to any date, an error message "end date must not be empty" occurs if the child platform/device has open end as well.

Steps to reproduce

• Create a configuration
• Create a platform
• Create a device
• Mount platform to configuration, leave end date empty (open end)
• Mount device to platform, leave end date empty (open end)
• Edit mount information of platform -> select an end date
• read the error message: "end date must not be empty"

What is the current bug behavior?

• error message "end date must not be empty" is rather inaccurate here

What is the expected correct behavior?

• display a more suitable message here

ToDo's

• Bug reproduced
• Cause found
• Bug fixed