fix NPE in external link icon loading

Signed-off-by: tobiaskaminsky <tobias@kaminsky.me>

src/main/java/com/owncloud/android/ui/activity/DrawerActivity.java

@@ -1034,10 +1034,16 @@ public abstract class DrawerActivity extends ToolbarActivity implements DisplayU
                     @Override
                     public void onLoadFailed(Exception e, Drawable errorDrawable) {
                         super.onLoadFailed(e, errorDrawable);
-                        mNavigationView.getMenu().findItem(getIdMenuItem()).setIcon(errorDrawable.getCurrent());
-                    }
-
+                        MenuItem menuItem = mNavigationView.getMenu().findItem(getIdMenuItem());
 
+                        if (menuItem != null) {
+                            if (errorDrawable != null) {
+                                menuItem.setIcon(errorDrawable.getCurrent());
+                            } else {
+                                menuItem.setIcon(R.drawable.ic_link_grey);
+                            }
+                        }
+                    }
                 };
 
                 DisplayUtils.downloadIcon(this, link.iconUrl, target, R.drawable.ic_link_grey, size, size);