Maybe try to think of the phrase as "to have open doors" :)
In Slovak we can actually use both "to be" and "to have" is this type of sentences. I never thought about it until I saw this Swedish sentence..
That being said, many people colloquially treat them as normal neuter words and say museumet and museumen. Or mix the two paradigms and say something like museet in the definite but museum in the plural.