app.x is not initialized, so it defaults to empty.
fx = (app.x)^2 + app.x;
value^2 is only valid if the value is empty or scalar or a square matrix. We cannot tell which from that alone.
difffx = diff(fx,app.x,1);
The three-parameter version of diff requires that the second parameter be scalar or empty. If app.x is numeric scalar then fx is numeric scalar, and the second parameter would be the number of differences to take; with fx being scalar, if that does not outright fail, the diff is going to return empty unless app.x happens to not be a positive integer, in which case error. If app.x is empty then it would be treated as 1 but since fx would be empty in that case, you would get empty anyhow. So if app.x is numeric then the diff is either going to error (not empty or positive integer) or return empty.
app.resEditField.Value = subs(difffx,app.x,app.numEditField.Value);%%%%%%%this is the place where i got error
If app.x is empty or numeric then if we got here then subs(empty) is going to be empty and that would not be scalar and you need a scalar at that point.
So... what if app.x just happens to store a scalar symbolic variable? Then the diff() would work and produce a scalar symbolic expression, and the subs would probably work and produce a scalar symbolic expression.
So what is the problem in that case? This: you cannot set that kind of field to a symbolic expression, it is the wrong data type. You need to convert the result to the subs() into a numeric datatype. Like double(subs(...))
