don't crash when launching an app that isn't installed

This fixes the following crash:

* Install an app form F-Droid
* go to home screen
* uninstall app
* quickly switch to F-Droid the button will still show 'run' for a few
seconds
* launch the app you just uninstalled

app/src/main/java/org/fdroid/fdroid/AppDetails2.java

@@ -736,7 +736,12 @@ public class AppDetails2 extends AppCompatActivity
     @Override
     public void launchApk() {
         Intent intent = getPackageManager().getLaunchIntentForPackage(app.packageName);
-        startActivity(intent);
+        if (intent != null) {
+            startActivity(intent);
+        } else {
+            // This can happen when the app was just uninstalled.
+            Toast.makeText(this, R.string.app_not_installed, Toast.LENGTH_LONG).show();
+        }
     }
 
     @Override