Display issue when drawing self created widget


i have created my own widget and have an display issue. Its a label like widget to draw some data( a spinlabel)
When there are no digits drawed, there are two big rectangles in the screen. Like the rectangles MS Windows displays if there is a unknown character for example.
I uploaded a screen of my problem. Display of my application
Does anyone knows if i can fix this? Its very annoying…

To create my own widget i took the lv_img.c as base