Okay, so it says that the error is happening at line 152 of the ItemSlab class which is in the makeState() method.
The line that is failing is: return this.doubleSlab.getDefaultState().withProperty(p_185055_1_, (T)p_185055_2_);
The first parameter p_185055_1_ is of type IProperty and the second parameter p_185055_2_ is of type Comparable<?>.
The error is a null pointer exception which means that one of the parts of the line is unexpectedly a null. It could be the parameters, but could also be the object like the doubleSlab itself or the default state returned.
So the next step to debug is to use the debug mode of your IDE (Eclipse if that is what you're using). You need to set a breakpoint on line 152 in the ItemSlab class and it should trigger when you try to create a double slap. Then you can inspect all the parts of the statement to see what value they have and if any are null.
You can also try to guess what the problem is. For example, it looks like the ItemSlab class is looking for a field called doubleSlab. Have you properly made a double slab in that field in your class?