It appears the Javascript alert message somehow concatenates the URL and ends up to show the invalid URL as such
"http://java.com/javafxjava.com/javafx"

See attached screenshot for detail

*** Test configurations:
- x86- Win 7, Vista
- jre; 7fcs
- fx: latest b45 runtime


*** Steps to reproduce
0) Install jre7
1) Make sure the system has not javafx runtime installed
2) Invoke any b45 fx sample applet
3) Click 'click to launch this app as webstart'
4) Try to cancel the fx runtime download

If you see the URL "http://java.com/javafxjava.com/javafx" mentioned in the Javascript Alert dialog (as in the screeshot), then the issue is reproducible