[LVGL] Slider value doesn't warn about an empty variable name

[OdaiJ]

Describe the bug
When creating a slider with an event handler, and giving empty input to its "Value" variable name, no error or warning is thrown. After building, a set_var_() function is used in the screens.c callback function.

To Reproduce
Steps to reproduce the behavior:

1. Launch EEZ Studio 0.26.2.
2. Create a v9.2.2 LVGL project.
3. Place a slider.
4. In its properties, click on the event handlers + icon.
5. Add a new action - I added the action whatever
6. Select VALUE_CHANGED event (I know, doesn't make sense, but it's 02:00 in the morning and I am so tired).
7. Leave user data as 0.
8. Head up to "SPECIFIC" in the slider's properties, and select "Variable" in "Value".
9. Remove the default "25" in the "Value" input box. (Notice, it's not giving a warning.):

10. Check and build - notice no warnings or errors.
11. Head to the build directory and check screens.c:

Expected behavior
When giving an empty input, throw an error or a warning, like when entering an invalid variable name:

Btw, this builds regardless of the error/warning too? Maybe a bug also?

Screenshots
Included with steps.

Desktop (please complete the following information):

• OS: Windows
• Version: 11

LVGL version (if used)

• 9.2.2

Disclaimer
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[mvladic]

Fixed. Thanks!