And handle the null case in the constructor itself. Closes #1794 (again)

src/main/java/net/minecraftforge/fluids/FluidStack.java

@@ -23,7 +23,12 @@ public class FluidStack
 
     public FluidStack(Fluid fluid, int amount)
     {
-        if (!FluidRegistry.isFluidRegistered(fluid))
+        if (fluid == null)
+        {
+            FMLLog.bigWarning("Null fluid supplied to fluidstack. Did you try and create a stack for an unregistered fluid?");
+            throw new IllegalArgumentException("Cannot create a fluidstack from a null fluid");
+        }
+        else if (!FluidRegistry.isFluidRegistered(fluid))
         {
             FMLLog.bigWarning("Failed attempt to create a FluidStack for an unregistered Fluid %s (type %s)", fluid.getName(), fluid.getClass().getName());
             throw new IllegalArgumentException("Cannot create a fluidstack from an unregistered fluid");