title: VSIX LanguageService FindToolWindow(...) returns null
body: <p>I have two VSIX packages (VS2010 SDK), one containing a LanguageService and one the ProjectPackage (based on Managed Package Framework for Projects) for my language. Now I need to establish some sort of communication channel where the LanguageService can retrieve additional information from the ProjectPackage. In the ProjectPackage, the information is available via a toolWindow, which can be retieved using</p>
  
  <pre><code>package.FindToolWindow(typeof(HierarchyViewerWindow), 0, false) as HierarchyViewerWindow;
  </code></pre>
  
  <p>The LanguageService inherits from the same Package base Class as the ProjectPackage, so I thought I could use the same mechanism here. The call returns <code>null</code> though.</p>
  
  <p>So the basic question is whether I am doing something wrong or this is impossible for some reason... Or if there is a better way to establish communication between a LanguageService and another VSIX package.</p>