Please share the exact Forms version you are using. If you are not using 188.8.131.52, you should consider using it.
Please also share the exact line of code that is failing (the SET_WINDOW_PROPERTY line and related code). Also include the complete error message and its text.
If you are generating on the command line, it might be easier to troubleshoot from within the Builder. Open the offending module in the Builder then press CTRL+Shift+K. This should expose the same error and exactly where in the code it is having the problem. That should help you to better understand what it is going on.
And most importantly, if you are upgrading an app from 6 to 12, it is very important that you read the Forms 6i to 12c Upgrade Guide as it explains the differences between 6 and 12. It also explains what code you may need to update.
are you compiling or running a Form? Usually these FRM-Errors come up during runtime.
Dpending on what you are doing (calculating positions with Windows) you can get to negativ values in 12c.
Example: get_window_property Returns 0,
set_window_property becomes negative
The best attempt to clarify this to is activate variables in the debug tab canvas of the forms builder, then set a debug point before this calculation and start forms in debug mode (one button debug). The Debugger in 184.108.40.206 is the first stable 64 Bit Debugger and very powerful and useful to detect these Errors.
Make sure you have IE11 with a 32 Bit Java plugin on your Client and a preference to your Formsserver like this:
Debug break Points are made with a double-click in the Forms PL/SQL Editor on the left line like this
When the Form started in debug Modus you will see a flashing Forms builder and the active debug Windows.
Step through the Code with
and watch the values that become negativ or invalid
I guess you have trouble with the calculations of the "forms-mdi-window"