The Ninja product manuals caution against processing hot foods or hot liquids. Blending hot foods or hot liquids in a blender can be dangerous, and it is safer to let them cool down before blending.
According to the FAQs and product manuals for various blender models on the Ninja support page, one should try to avoid blending hot soup in a Ninja blender. In the FAQs, Ninja recommends that letting the ingredients cool down before blending them. In the manuals, Ninja cautions against blending hot foods or liquids, and also cautions against exposing the blender container to extreme temperature changes.