Fix exception when deleting extension with hidden components

Change-Id: Id3df87c907e20a3cc1241337a64594ca48c6ad29

appinventor/appengine/src/com/google/appinventor/client/editor/youngandroid/palette/YoungAndroidPalettePanel.java

@@ -316,8 +316,10 @@ public class YoungAndroidPalettePanel extends Composite implements SimplePalette
   public void removeComponent(String componentTypeName) {
     String categoryString = COMPONENT_DATABASE.getCategoryString(componentTypeName);
     ComponentCategory category = ComponentCategory.valueOf(categoryString);
+    if (simplePaletteItems.containsKey(componentTypeName)) {
       removePaletteItem(simplePaletteItems.get(componentTypeName), category);
     }
+  }