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

@@ -143,7 +143,10 @@ public class TextInputLayoutHelper extends TextInputLayout
 	 */
 	public void setHelperText(CharSequence text)
 	{
-		mHelperText.setText(text);
+		if (mHelperText != null)
+		{
+			mHelperText.setText(text);
+		}
 	}
 
 	private void showHelper(boolean show)