Make sure show return nothing.

src/animation.jl

@@ -103,6 +103,7 @@ function Base.show(io::IO, ::MIME"text/html", agif::AnimatedGif)
     else
         error("Cannot show animation with extension $ext: $agif")
     end)
+    return nothing
 end
 
 

src/output.jl

@@ -197,6 +197,7 @@ for mime in ("text/plain", "text/html", "image/png", "image/eps", "image/svg+xml
           prepare_output(plt)
           _show(io, m, plt)
         end
+        return nothing
     end
 end