Fix undefined projection_matrix variable

[schillic]

This fixes the first error in #707. It still crashes at a later problem.

Example from the notebook:

A = rand(4, 4)
S = LinearContinuousSystem(A)
X0 = rand(Hyperrectangle, dim=4)
M = rand(1, 4)  # note that this must matrix be 1D
P = InitialValueProblem(S, X0)
opts = Options(:T=>1.0, :project_reachset=>true, :projection_matrix=>M)
opts_algo = Options(:δ=>0.1, :vars=>[1, 2])
solve(P, opts, op=BFFPSV18(opts_algo))