Update isocorgui.py

for issue #25 : remove empty string ('') for the tk.StringVar() call that make iscocor startup failed with newer version of the *tk* library.

isocor/ui/isocorgui.py

@@ -599,7 +599,7 @@ def createWidgets(self):
         self.purityManager.config(scrollregion=self.purityManager.bbox("all"))
 
         tracer_list = list(self.cleanDfIsotopes['subscriptName'])
-        self.isotopictracerEntered = tk.StringVar('')
+        self.isotopictracerEntered = tk.StringVar()
         self.isotopictracerlbl = ttk.Label(
             optionFrame, text="Isotopic tracer")
         self.isotopictracerCBB = ttk.Combobox(