fix boundary delete (#4175)

Author: gmalinve

pyaedt/modules/Boundary.py

@@ -94,9 +94,9 @@ def delete(self):
             self._app.o_maxwell_parameters.DeleteParameters([self.name])
         else:
             self._app.oboundary.DeleteBoundaries([self.name])
-        for el in self._app.boundaries:
+        for el in self._app.boundaries[:]:
             if el.name == self.name:
-                self._app.boundaries.remove(el)
+                del self._app._boundaries[el.name]
         return True
 
     def _get_boundary_data(self, ds):