now your expression is optional, so your IsNull raster is your conditional raster where 1 is true and 0 is false, so you don't need and optional query your fs_ras is the raster it should use if there are null values and the false raster is the original raster.
Now the crux is the error messages. Did you explicitly set your environments tab on the bottom center-ish of the dialog (anticipating a .... noooo) if not, do so, set the extents the cell size and the snap raster... of course you should have done this for the two previously created rasters... always set everything in the tool, don't rely on global settings.
The folder, is one that exists? and for testing I use variants X as a filename, since X, X1, X2 etc since you aren't specifying a file extension (like tif), it will create an esri grid.
The old error list is still useful ArcGIS Help (10.2, 10.2.1, and 10.2.2) and if you go through the error numbers, it doesn't like something about your folder and or filename for the output grid... so have a check, try without the conditional statement. (if you ever have to use the SQL dialog...type little to nothing and let the dialog build it for you)
That's all I can think of now.