To reproduce you must have the Git and pullrequest plugins enabled:
1. Create a repository A
2. Fork the repository A into a repository B
3. Delete the repository A
4. Access to the repository B

Trace of the fatal error:
Fatal error: Call to a member function getFullPath() on a non-object in /usr/share/tuleap/plugins/pullrequest/include/pullrequestPlugin.class.php on line 221
Call Stack
# Time Memory Function Location
1 0.0000 662784 {main}( ) ../index.php:0
2 0.0331 4823200 GitPlugin->process( ) ../index.php:39
3 0.0492 8439600 PluginController->process( ) ../gitPlugin.class.php:608
4 0.0533 8604488 PluginController->executeViews( ) ../PluginController.class.php:236
5 0.0925 9871032 PluginViews->display( ) ../PluginController.class.php:185
6 0.0925 9871472 call_user_func_array ( ) ../PluginViews.class.php:60
7 0.0925 9871720 GitViews->view( ) ../PluginViews.class.php:60
8 0.0927 9878200 GitViews_ShowRepo->display( ) ../GitViews.class.php:227
9 0.0930 9901280 GitViews_ShowRepo_Content->display( ) ../ShowRepo.class.php:108
10 0.0930 9901280 GitViews_ShowRepo_Content->displayHeader( ) ../Content.class.php:111
11 0.0970 10083072 GitViews_ShowRepo_Content->getCloneUrl( ) ../Content.class.php:171
12 0.0977 10083288 GitViews_ShowRepo_Content->getAdditionalActions( ) ../Content.class.php:294
13 0.0977 10083968 EventManager->processEvent( ) ../Content.class.php:320
14 0.0977 10083968 EventManager->processEventOnListener( ) ../EventManager.class.php:77
15 0.0977 10083968 PluginProxy->processEvent( ) ../EventManager.class.php:91
16 0.0977 10083968 pullrequestPlugin->git_additional_actions( ) ../PluginProxy.php:57