WhenEvent and Resetting of Variable in PDE when operation succeeds

Question

I have had success in using WhenEvent to reset or change a variable within NDSolve with ordinary differential equations. My working, functioning MWE is:

Clear[usol, u, t, x, λ];
usol = NDSolveValue[
  {
   D[u[t, x], t] == λ[t, x] D[u[t, x], x, x],
   u[0, x] == 0,
   u[t, 0] == Sin[t],
   u[t, 5] == 0,
   λ[0, x] == 1,
   WhenEvent[t > 5, λ[t, x] -> 1.25*λ[t, x]]
   },
  u,
  {t, 0, 10},
  {x, 0, 5},
  DiscreteVariables -> λ]
Plot3D[usol[t, x], {t, 0, 10}, {x, 0, 5}, PlotRange -> All]
NIntegrate[usol[1, x], {x, 0, 5}]) (*This evaluates to 0.669665*)

However, I would like to use WhenEvent to perform an operation such as StopIntegration or changing a variable when a calculation such as area under the curve represented by the dependent variable attains a certain value. For this, I tried:

Clear[usol, u, t, x, λ];
usol = NDSolveValue[
  {
   D[u[t, x], t] == λ[t, x] D[u[t, x], x, x],
   u[0, x] == 0,
   u[t, 0] == Sin[t],
   u[t, 5] == 0,
   λ[0, x] == 1,
   WhenEvent[NIntegrate[usol[t, x], {x, 0, 5}] > 0.1, end = t; 
    "StopIntegration"]
   },
  u,
  {t, 0, 10},
  {x, 0, 5},
  DiscreteVariables -> λ]

However, I am beset with a slew of error messages in this latter case. The error message starts with:

The integrand usol[0,x] has evaluated to non-numerical values for all sampling points in the region with boundaries {{0,5}}

The other question here on WhenEvent and NDSolve for partial differential equations did not have similar errors.

I am not sure what is going wrong; the operation NIntegrate[usol[t, x], {x, 0, 5}] < 1 should have "ended" the NDSolve Integration at t=1 at the least as in my functioning MWE.