A Map Widget that is a child of a other widget (parent) with a priority higher then 0 will not get displayed.
Example of a broken layout with a broken MapWidget:
https://gyazo.com/ee9d0ea001d6b3a5e9d05bdf6a31de89
Example of a working layout with a working Map Widget:
https://gyazo.com/3a66f8bab48990b614f8909ba00fe478
The only difference on these examples is that the Parent Widget has the priority set to 1 instead of 0.