As you have mentioned, the phrase soft spot can be used to mean a sentimental vulnerability, weakness or affinity for a person or animal (capable of receiving emotional affection). For example, "I have a soft spot for those dogs in the shelter!"
It can also be used, although not commonly, to mean a physical vulnerability in an inanimate object. For example, "there was a soft spot in the network that allowed the hackers to enter."
However, in your final example of a weakness or affinity that a person can have towards an inanimate object is not very common. Although, as Andrew pointed out, if the inanimate object has emotional attributes and is capable of receiving sentimental affection, it can be used. So, you might say, "I have a soft spot for ice cream, it just looks so lonely sitting there in the freezer all by itself".
I hope that helps :)
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