Product with variations: Changing a Variation does not change the picture

I installed WP 4.7 and MarkePress 3.1.2 with theme Twenty Seventeen and tried a product with Variations of color. I like the way it is handled now when you enter a product in the backend (except the changing of the stock quantities is a mess, because you have to click at every single quantity - not in version 2.x, where you could just jump from one to another).
I ran into a problem, when I looked at the product page: It loads perfect with the default product and it's picture, and it has a list of the available colors.
But: when you change the color, the picture does not change - worse: it is replaced by the thumbnail_placeholder. Even changing back to the default color (as well as every other change of color) does not bring any picture back - only the thumbnail_placeholder is shown.
Some hours of debugging resulted in finding a programming error in includes/common/class-mp-product.php, function image_url , line 2243 - see the screenshot in the first comment, which shows the corrected version.
Even just reading this piece of code shows you, that the variable $img_url can never be "not empty" in line 2248, where it is finally decided that the thumbnail_placeholder will be returned.
Small typo - big effect :wink:
Happy New Year!