title: Unable to get property 'subscribe' of undefined or null reference
body: <p>I am experiencing an issue with Implementing knockout Js while subscribing a dropdown value change in my HTML Page.  Here is my HTML</p>
  
  <pre><code>    &lt;div&gt;
        &lt;span&gt;Request Types : &lt;/span&gt;
        &lt;select data-bind="options: requestTypes,
                      optionsText: 'Name',
                      optionsCaption: '--Select--',
                     value: selectedRequestType"&gt;
         &lt;/select&gt;
      &lt;/div&gt;
  </code></pre>
  
  <p>While the ViewModel is as shown below</p>
  
  <pre><code>                        wd.viewModel = function () {
                          var data = {};
                          var self = this;
                          var url = '';
                          var selectedRequestingCountry = ko.observable("");
                          self.optionsProvider = new wd.optionsProvider(self);
                          self.objGenralInfo = new wd.GI_form(data);
                          /* For Requesting Country */
                          url = wd.ConfigObject.baseUrl + wd.ConfigObject.masterServiceUrl + wd.ConfigObject.category.importingCountries;     // Fetch Requesting Countries     
                          self.importingCountries = self.optionsProvider.get("importingCountries", url, {});
                          self.selectedRequestingCountry = ko.observable("");
                          this.requestingCountryArea = ko.observable("");
  
                          /* For Generic Name*/
                          url = wd.ConfigObject.baseUrl + wd.ConfigObject.masterServiceUrl + wd.ConfigObject.category.genericNames;     // Fetch Generic Names     
                          self.genericNames = self.optionsProvider.get("genericNames", url, {});
                          self.selectedGenericName = ko.observable("");
  
                          /* Override Requesting Country Address */
                          self.IsOverride = ko.observable(false);
  
                          /* For Request Types */
                          self.selectedRequestType = ko.observable("");
                          self.selectedTradeName = ko.observable("");
                      };
  
                      // Whenever the selected Generic Name changes, populate the Request Type
                      var myName = wd.viewModel.selectedGenericName.subscribe(function () {
                          wd.viewModel.url = wd.ConfigObject.baseUrl + wd.ConfigObject.masterServiceUrl + wd.ConfigObject.category.requestTypes + "/" +
                                wd.viewModel.selectedGenericName().Name;
                          wd.viewModel.requestTypes = wd.viewModel.optionsProvider.get("requestTypes", url, '');                                        // Fetch request types based on generic name
                      }, wd.viewModel);
  
                      $(function () {
                          ko.applyBindings(wd.viewModel);
                      });
  </code></pre>
  
  <p>Request Type dropdown values are populated only after Generic Name Drop down values are loaded.User has to select the Generic name first. On Page load, it throws an error " Unable to get property 'subscribe' of undefined or null reference".</p>
  
  <p>Any help is much appreciated as I am new to knockout javascript MVVM framework.</p>