Question: Error, (in plots/odeplot) input is not a valid dsolve/numeric solution

it seems impossible to draw all DE system in one time.

quite easy to get error when can not solve

> newsys := [Diff(x1(t), t) = VectorCalculus[`-`](VectorCalculus[`*`](2, x3(t)^2)), Diff(x2(t), t) = VectorCalculus[`-`](u(t)), Diff(x3(t), t) = u(t), Diff(u(t), t) = 0];
[ d 2 d d
[--- x1(t) = -2 x3(t) , --- x2(t) = -u(t), --- x3(t) = u(t),
[ dt dt dt

d ]
--- u(t) = 0]
dt ]
> DEplot3d(value(newsys), [x1(t), x2(t), x3(t), u(t)], t = 0 .. 1, [[x1(0) = 1, x2(0) = 1, x3(0) = 1, u(0) = 1]], scene = [x1(t), x2(t)]);
Error, (in DEtools/DEplot) 'scene' must specify three variables in DEplot3d: scene = [x1(t), x2(t)]
> Sol := dsolve(`union`(newsys, {u(0) = .5, x1(0) = .5, x2(0) = .5, x3(0) = .5}), range = 0 .. 15, numeric); plots:-odeplot(Sol, [x1(t), x2(t), x3(t), u(t)], t = 0 .. 15);
Error, invalid input: `union` received [Diff(x1(t), t) = -2*x3(t)^2, Diff(x2(t), t) = -u(t), Diff(x3(t), t) = u(t), Diff(u(t), t) = 0], which is not valid for its 1st argument
Error, (in plots/odeplot) input is not a valid dsolve/numeric solution