fix inout validation on first row

gui/platform_edit.ui

@@ -446,6 +446,9 @@
    </layout>
   </widget>
   <widget class="QPushButton" name="add_row_but">
+   <property name="enabled">
+    <bool>false</bool>
+   </property>
    <property name="geometry">
     <rect>
      <x>532</x>

src/platform_edit.py

@@ -77,6 +77,7 @@ class PlatformEdit(QWidget):
             check_box.checkStateChanged.connect(self.check_valid)
 
         self.first_tier_box.valueChanged.connect(self.check_valid)
+        self.first_tier_fee_box.valueChanged.connect(self.check_valid)
         self.first_tier_box.valueChanged.connect(self.update_tier_labels)
 
         # NOTE: Signal defined in UI file to close window when save button clicked