Generally on scroll changed work on ListView..

So GetChildView() will not work as you are calling in the listener of listview

You can try this method outside of your listener and use the value inside the listener...

check if bottom reached method of scrollview
Maple can  separate and eliminate a function in a system of PDE equations, with casesplit in the PDEtools package. How to do that in Mathematica? Starting PDEs are:

$$ -A\frac{\partial ^2Q(x,y)}{\partial x^2}+B\frac{\partial ^3P(x,y)}{\partial x^3}+C Q(x,y)-C\frac{\partial P(x,y)}{\partial x}+D\frac{\partial ^2Q(x,y)}{\partial y^2}=0 $$

$$ -B\frac{\partial ^3Q(x,y)}{\partial x^3}+E\frac{\partial ^4P(x,y)}{\partial x^4}-C \frac{\partial Q(x,y)}{\partial x}+C\frac{\partial ^2P(x,y)}{\partial x^2}-F\frac{\partial ^2P(x,y)}{\partial y^2}=0 $$

Maple code 

des:= {-A*diff(Q(x,y),x,x)+B*diff(P(x,y),x,x,x)+C*Q(x,y) 
 -C*diff(P(x,y),x)+D*diff(Q(x,y),y,y)=0, 
 -B*diff(Q(x,y),x,x,x)+E*diff(P(x,y),x,x,x,x)-C*diff(Q(x,y),x) +
  C*diff(P(x,y),x,x)-F*diff(P(x,y),y,y)=0};

PDEtools:-casesplit(des,[Q,P]);


Maple output is

$$\left( -{B}^{2}+EA \right) {\frac {\partial ^{6}}{\partial {x}^{6}}}P \left( x,y \right) + \left( -  D  C-AF \right) {\frac {
\partial ^{4}}{\partial {y}^{2}\partial {x}^{2}}}P \left( x,y \right) 
+ \left( AC-CE \right) {\frac {\partial ^{4}}{\partial {x}^{4}}}P
 \left( x,y \right) +  D  F{\frac {\partial ^{4}}{
\partial {y}^{4}}}P \left( x,y \right) -  D E   {
\frac {\partial ^{6}}{\partial {y}^{2}\partial {x}^{4}}}P \left( x,y
 \right)   +CF{\frac {\partial ^{2}}{\partial {y}^{2}}}P
 \left( x,y \right) = 0
$$
