A style expression of the wrong type hides the entire layer

• `Error: layers.districts.paint.fill-color: Expected number but found string in expression: ["get", "level"]`
• `Error: layers.districts.paint.fill-opacity: "level" is not a valid expression`
• `map.queryRenderedFeatures({ layers: ['districts'] })` returns an empty array while `map.getStyle().layers` contains the layer

1. List layer ids from `map.getStyle().layers` and confirm `map.getLayer('districts')` is not undefined
2. Replace the paint expression with a constant color and see whether features appear
3. Check whether a property name was written as a bare string instead of `["get", ...]`
4. Wrap the value in `["to-number", ["get", ...]]` and retest
5. Confirm the property exists and inspect its type in the source data

Use array expressions with explicit conversion: `'fill-color': ['interpolate', ['linear'], ['to-number', ['get', 'pop_density']], 0, '#e8f4ff', 5000, '#8ec4ff', 20000, '#1a6fd4']` and `'fill-opacity': ['case', ['==', ['get', 'level'], 'city'], 0.75, 0.45]`.

Error: layers.districts.paint.fill-color: Expected number but found string
  in expression: ["get", "level"]

Error: layers.districts.paint.fill-opacity: "level" is not a valid expression

• Writing `"level"` as a literal string in expression context and expecting it to be read as a property name
• Feeding a string property to `fill-opacity`, which drops the whole layer instead of only the offending features