Both the errors should be handled differently.
Form builder crashes when tried to open a form (.FMB)
1) This could be related to version mismatch between the FMB and form builder. Forms developer version should match the Oracle Application version.
2) Make sure all the suggested Oracle developer edition patches are applied to match with desktop OS (32/64 Bit).
Form builder crashes while opening Canvas
1) Canvas physical width and height should be in the limit of run time Window.
2) Open other canvases from same form or different forms and then open the canvas that's failing.
3) Place the form in directory where TEMPLATE.fmb and other dependent forms exists along with PLL's and try opening the canvas.
4) Delete all the Graphics from canvas and open the canvas.
