add effects after a Nul object

That is a perfect example of a tutorial prepared by an enthusiast that uses inefficient workflows and gives some pretty poor advice. The first piece of bad advice happens at 2:24, scaling the black solid because it is going to be moving when it is moving with the nulls is not a good idea. The Layer Settings should have been opened up and the size of the solid increased instead of scaling the layer because for several reasons that I won't go into right now. The second poor technique happens at about 3:00 when the presenter uses parenting the Black Solid to a null when you want to move the Origin. Advanced Lightning has a position property for the Origin and Direction, the null has a position property. A better technique would be to use an expression to attach the Origin to the Null. If you set a keyframe for Origin then selected the Null you would see pickwhip options right in the Parent column and you could easily drag from Origin to the Null's position. Here's another minor bit of laziness from the presenter. None of the layers are named so it becomes really easy to get lost. The rest of the tutorial follows the same inefficient way of adding the other elements like the finger glows, completely ignores blend modes which always enhance this kind of energy effect, and relies on a bunch of duplicate layers that make final adjustments extremely tedious. The basic layering of the effects is a decent idea, but the implementation is very amateurish.  

I'm not sure where your problem with the Curves effect is because I can't see the modified properties of the layer with the Curves applied and you have not soloed the layer and tried different settings.

If you want to keep the same ideas but greatly simplify and improve the workflow take a look at this tutorial that I just prepared. All of the effects used to create and control the energy beam are on one layer. I see lots of questions about tying effects to nulls or other layers and this workflow will work for many of them.

It may take a few minutes to process the HD and 4K versions of this video.

I am assuming that Layer 4 (selected) in your screenshot is a comp sized solid that is the child of Layer 7, a null that was attached to one of the fingers by using AE's motion tracker. It looks like you have Advanced Lightning, then Glow, then Curves applied and you are trying to adjust the Green Channel. 

If you create a new comp, add a solid, then apply all three of those effects in order and leave them all at their default settings you should be able to grab the green curve and make changes to the image. If that test works then reset all of the Effects applied to Layer 4 and see if that works. If it does, then one of the settings in one of the effects you added is wrong. If that test does not work then check GPU settings in the Project Panel. If you still can't figure it out we need more info.

You can find everything that was changed in layer 4 by selecting the layer and pressing the U key twice. This will let you see every change you made in the timeline so you can start resetting the controls there to figure out which one is causing the problem.

This problem may not be related to the tutorial you are watching but it would really help us to help you if you posted a link to the tutorial. Many of the ones you find on YouTube are poorly presented and ill-conceived. It's always better if we know what you were trying to do.

Are you mixing up null layer with adjustment layers?

*Martin

It sounds like you are new to AE and that you are trying to use After Effects without spending enough time with the User Guide. 

Because there are hundreds if not thousands of possible workflows that involve some kind of tracking and null layers it is difficult if not impossible to figure out what you are trying to do without some kind of description of your workflow and design goals. I have a few minutes that I don't need to use for anything else so I will take a guess. 

You have something moving in the frame that you want to Motion Track so you added a Null to the timeline, then selected the footage layer. If you were following the recommended workflow you had the Motion Tracking workspace open so you would have easy access to all of the tools you need. With the Motion Tracking workspace open, you made sure that the footage layer you wanted to track was selected (active) in the timeline and verified that it was selected as the Motion Source in the Tracker Panel. This opens the selected footage layer in the Layer Panel, which looks a lot like the Composition Panel except that there is now a Time Ruler that is the same color as the label color of the layer in the timeline at the bottom, some Alpha Overlay icons, some time displays, and a View Options Drop Down menu at the bottom and a Render checkbox.  You then ran the motion tracker, set the Null as the target if it was not already properly identified, and applied the Tracker data to the Null. With the Footage layer still selected in the Timeline you chose the Curves effect from the Effects menu and added it to the selected footage layer. This opens up the Effects Control Panel and displays the Curves GUI. You then make some adjustments but you do not see any changes in the footage.

If that is your workflow, and it should be up to this point, the Footage Panel is still active and the Composition Panel Tab is to the left of the Layer Panel Tab at the top of their Panel Group. When you make curve adjustments the changes to the layer will not be visible in the Layer Panel unless the Render checkbox at the bottom right of the Layer Panel has been activated. The normal workflow would be to go back to the Composition Panel or change to the Color workspace, which would be the recommended workspace for making color adjustments. Changing workspaces would switch you back to the Comp Panel so you could work on your comp and see the changes in the composition. 

There is no reported bug so I am guessing that you are simply not following the recommended workflow and you didn't know what the Render option in the Layer Panel was for.

If that does not solve your problem then you have committed some other user error and we need design goals, workflow details, screenshots that show us what you have changed on the layers that are giving you problems (press 'uu' to find them), footage details, and OS, system, and AE version details.

I'm not clear what you are asking. What would/ should applying effects to a Null layer even do, considering that they do not contain any visible pixels? You're not really making sense and your description/ explanation is just vague and confusing bumbling, no offense. You need to do better and tell us exactly what you were hoping to do.

Mylenium