Use of local result variables when calling control/automation

What causes this issue?

You cannot use a local variable in a call to a control or automation
server.
For performance reasons, the system collects calls to front end
components, such as controls, in a queue and does not transfer them to
the frontend for processing until the so-called flush. These calls are
processed in the front end asynchronously to calls in the ABAP source
code. If local variables are used in calls to controls for including
the return values (for example the selected node of a tree), it is
possible that the local variable will not exist at the time of the
flush. It will not be possible to transfer the return value to the
variable.

System Response

It is highly unlikely that further processing will continue correctly
without return values. For safety reasons, further processing is
terminated.

How to fix this error?

Check your application for local variables that are used for return
values of control/automation calls.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.