An error when performing multiplication of noncommutative polynomials

When running the following code with `DynamicPolynomials.jl v0.6.6`:
```
using DynamicPolynomials
@ncpolyvar x[1:3]
Delta = x[1]^2 + x[2]^2 + x[3]^2
Delta*x[1]
```
I got

> ERROR: MethodError: no method matching map_exponents(::typeof(+), ::MonomialVector{…}, ::Variable{…})
The function `map_exponents` exists, but no method is defined for this combination of argument types.