Go to Preferences -> Run/Debug -> Perspectives, select "Always" for "Open the associated perspective when an application suspends".
I just tried choosing no and then changing it here. It worked for me.
If it doesn't work for you, maybe you can try hitting on "Restore Defaults" in the Run/debug -> Perspectives page.
I tried again, and here's what happened.
I still get the error saying "The currently displayed page contains invalid values." and the view does not change. I have to click on another category in the Run/Debug section, then I can click on Perspectives to see it's options. When the view changes, both sets of radio buttons for "Open the associated perspective when launching" and "Open the associated perspective when an application suspends" have nothing selected. So I changes the two options hit Okay and it seems to work. But if I go back to Run/Debug -> Perspectives I have to again click on another option in Run/Debug before I can view the options for Perspective, and again both sets of radio buttons have nothing selected.
Is there something in one of the Workspace/.settings files that scrogged or should I file a bug again Flash Builder for this?
That preference page is from Eclipse. I suspect your settings file is messed up.
I have tried the same flow in a clean workspace and have not experienced problems.
But if you can reproduce this (hitting no on suspend and then trying to change the preference) with a clean workspace, file a Flash Builder bug and we'll follow it up with Eclipse.
I tried creating a new Workspace and went right to the Run/Debug -> Perspectives and got the same error. I'll file a bug for it. Thanks.