to handle – Problem with a small program: Error: "Charting Private pvar"

I am creating a smaller standalone application. I've created a small program element, which needs to be examined in the end. However, sometimes, when I change a portion of the entry for "request" or "offer", it does not change the output, but generates a lot of errors. On the other side – it works exactly as desired sometimes. I think this could be a problem with the graph. Please, help me repair my program (try to change the entry several times)! The code is here:

DynamicModule[{Demand, Supply, G, F, X1, Y1, Y2, Graf, Slutinterval,CS, PS, SS}, Manipulate[
G = Q /. Solve[Supply == Demand, Q][[1]];
F = Block[{Q = G}, Supply];
CS = N[Integrate[Demand, {Q, 0, G}] - (F * G)];
PS = N[(F*G) - Integrate[Supply, {Q, 0, G}]];
SS = CS + PS;
X1: = G / 6;
Y1: = ((Block[{Q = 0}, Demand]) - F) / 2 + F;
Y2: = F - ((F - (Block[{Q = 0}, Supply])) / 2);
Slutinterval: = G + G / 100 * 30;
Column[{
StringForm["Først bestemmes ligvægtsmængden og prisen. Dette gøres ved at finde Supply = Demand:"],
StringForm["Supply = Demand [DoubleLongRightArrow] Solve[`1` == `2`,Q] = `3`", supply, demand, G],
StringForm["Q = ``", G],
StringForm["På samme vis bestemmes den tihørende optimale pris:"],
StringForm["P = Supply(``) = ``", G, F],
StringForm["Nu kan henholdsvis Consumer Surplus, Producer Surplus, og Social Welfare bestemmes:"],
StringForm["CS = Integrate[Demand,{Q,0,Q}] - (P * Q) = Integrate[`1`,{Q,0,`2`}] - (`3` *` 2`) = `4`", request, V, F, CS],
StringForm["PS = (P * Q) - Integrate[Supply,{Q,0,Q}] = (`1` *` 2`) - Embed[`3`,{Q,0,`2`}] = `4`", F, G, Supply, PS],
StringForm["SS = CS + PS = `1` + `2` = `3`", CS, PS, SS],
Show[(Plot[{Supply, Demand}, {Q, 0, Slutinterval},
   AxesLabel -> {"Q", "P"},
   ImageSize -> 700,
   PlotLabels -> {"Supply", "Demand", " "}]), (Ground[{Supply, Demand, F}, {Q, 0, G}, Filling -> 2 -> F, 
   PlotStyle -> {Orange, Blue, {Thick, Dashed, Black}},
   PlotLabels -> Placed[{"CS"}, {X1, Y1}]]), (Ground[{Supply, Demand}, {Q, 0, G}, Filling -> 1 -> F,
   PlotLabels -> Placed[{"PS"}, {X1, Y2}]]), (ContourPlot[{Q == G}, {Q, 0, Slutinterval}, {y, 0, F}, 
   ContourStyle -> {Black, Dashed, Thick}]), (ListPlot[{Labeled[{0, F}, "P"], Labeled[{G, 0}, "Q"], {G, F}}, PlotStyle -> PointSize[Large]])]}],
Style["Velfærdsteori ved markedsligevægt (FK)", 20, Bold],
Style["Denne funktion beregner ligevægtspris og mængde på et marked. Ydermere beregner og skitserer", 12, Plain],
Style["funktionen Consumer Surplus, Producer Surplus samt Social Surplus.", 12, Plain],
Control @ {{Demand, -Q + 98}, InputField, ImageSize -> Full},
Control @ {{Supply, 0.5Q + 2}, InputField, ImageSize -> Full}
]]