android: Fix null pointer dereference in TextInputLayoutHelper

The AndroidX/Material version of TextInputLayout actually supports a
helper text, but it is always shown, so we continue to use our version.

src/frontends/android/app/src/main/java/org/strongswan/android/ui/widget/TextInputLayoutHelper.java

@@ -142,9 +142,12 @@ public class TextInputLayoutHelper extends TextInputLayout
 	 * @attr ref R.styleable#TextInputLayoutHelper_helper_text
 	 */
 	public void setHelperText(CharSequence text)
+	{
+		if (mHelperText != null)
 		{
 			mHelperText.setText(text);
 		}
+	}
 
 	private void showHelper(boolean show)
 	{