[Bug] Data Source picker always disabled

**Describe the bug**
The layer data source picker is always disabled

**Expected behavior**
The data source picker should be visible and enabled when there are multiple datasets

**Additional context**
It seems that the data source picker is always disabled because of this check `layer.type && config.columns`. Since most layers have required columns, it's not possible to change the data source for a layer, even if the default chosen is not at all suitable for that layer's data.

Additionally, i'm getting the warning `Warning: Failed prop type: Invalid prop 'disabled' of type 'object' supplied to 'ItemSelector', expected 'boolean'.`
Perhaps this is why it's evaling to true?