### TypeError when trying to read a string entered in a graphical user interface and convert it to an expression

Hi!

I am trying to read a string entered by a user in a GUI constructed with tkinter, and convert this string to a function. However, Sage rises a TypeError when I do this. A minimal example follows.

```
import tkinter
def compute_phaser_polynomials():
var('omega')
str_GroupDelaySpec = entry1.get() # string with the expression of the group delay as a function of omega
#str_GroupDelaySpec = 'omega + 5/2'
print(str_GroupDelaySpec)
print(type(str_GroupDelaySpec))
gds.<omega> = QQ[]
group_delay_spec(omega) = gds(str_GroupDelaySpec)
print(group_delay_spec(omega))
print(type(group_delay_spec(omega)))
root = tkinter.Tk()
entry1 = tkinter.Entry(root)
entry1.grid()
button1 = tkinter.Button(root, text="Compute phaser polynomials", command=compute_phaser_polynomials)
button1.grid(row=1)
root.mainloop()
```

When tkinter is not involved, the conversion works.

```
gds.<omega> = QQ[]
f(omega) = gds('omega + 5/2')
print(f(omega))
print(f(omega).parent())
print(type(f(omega)))
Out: omega + 5/2
Out: Symbolic Ring
Out: <type 'sage.symbolic.expression.Expression'>
```

Am I doing something wrong? Is this a bug?

Thanks!