title: "TypeError: Cannot read property 'value' of null" in on_change event
body: <p>I am finding the documentation for openerp 7 to be frustrating.</p>
  
  <p>I am attempting to call a function from an on_change event to perform a calculation and place the result into another field within the same row.</p>
  
  <pre><code>from openerp.osv import osv, fields
  
  class degree_day(osv.osv):
  
  _name = "degree.day"
  _columns={
      'date': fields.date('Date'),
      'high_temp': fields.integer('High Temp'),
      'low_temp': fields.integer('Low Temp'),
      'heat_degree_day': fields.integer('Heat Degree Day' ),
      'hw_degree_day': fields.integer('HW Degree Day' ),
  }
  
  def generate_degree_day(self, cr, uid, ids, high_temp = 0, low_temp = 0, context=None):
      """ On change of temperature generate degree day numbers 
      @param high_temp: The day's high temperature
      @param low_temp: The day's low temperature
      """
      if not (high_temp and low_temp):
          return
  
      temp = 65 - (high_temp + low_temp) / 2
  
      if temp &lt; 0:
          temp = 0
  
      heat_degree_day = temp
      hw_degree_day = temp + 5
  </code></pre>
  
  <p>&lt;></p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;openerp&gt;
  &lt;data&gt;
      &lt;record id="degree_day_tree" model="ir.ui.view"&gt;
          &lt;field name="name"&gt;degree.day.tree&lt;/field&gt;
          &lt;field name="model"&gt;degree.day&lt;/field&gt;
          &lt;field name="arch" type="xml"&gt;
              &lt;tree string="Degree Day List" editable="bottom"&gt;
                 &lt;field name="date"  /&gt;
                 &lt;field name="high_temp" on_change="generate_degree_day(high_temp, low_temp)" /&gt;
                 &lt;field name="low_temp" /&gt;
                 &lt;field name="heat_degree_day"  /&gt; 
                 &lt;field name="hw_degree_day" /&gt;
              &lt;/tree&gt;
          &lt;/field&gt;
      &lt;/record&gt;
  
      &lt;record id="show_degree_day" model="ir.actions.act_window"&gt;
          &lt;field name="name"&gt;Degree Day&lt;/field&gt;
          &lt;field name="res_model"&gt;degree.day&lt;/field&gt;
          &lt;field name="view_type"&gt;form&lt;/field&gt;
          &lt;field name="view_mode"&gt;tree,form&lt;/field&gt;
      &lt;/record&gt;
  
  
      &lt;menuitem name="Degree Day" id="menu_degree_day" action="show_degree_day"/&gt;
  
  &lt;/data&gt;
  &lt;/openerp&gt;
  </code></pre>