[FEM] fix bug in potential constraint task panel

- fix logic of a checkbox

src/Mod/Fem/Gui/Resources/ui/ElectrostaticPotential.ui

@@ -90,6 +90,9 @@
      <property name="text">
       <string>Vector Field</string>
      </property>
+     <property name="checked">
+      <bool>true</bool>
+     </property>
     </widget>
    </item>
    <item row="2" column="0">

src/Mod/Fem/femtaskpanels/task_constraint_electrostaticpotential.py

@@ -81,6 +81,7 @@ class _TaskPanel(object):
             self._obj.AV_im_2_Disabled and \
             self._obj.AV_im_3_Disabled:
             self._vectorField_visibility(False)
+            self._paramWidget.vectorFieldBox.setChecked(False)
         QtCore.QObject.connect(
             self._paramWidget.vectorFieldBox,
             QtCore.SIGNAL("toggled(bool)"),