Monotouch Application fatal error

I am trying to understand the reason my app crashes, randomly it seems, when trying to present a view. The button click code:

if ( d.rootNavigationController.RespondsToSelector(
            new MonoTouch.ObjCRuntime.Selector("presentViewController:animated:completion:")))
        {
            d.rootNavigationController.PresentViewController(childBroswer, true,null);
        }
        else
        {
            d.rootNavigationController.PresentModalViewController(childBroswer, true);
        }

This is the stack trace:

at MonoTouch.UIKit.UIApplication.Main (string[],string,string) [0x0004c] in /Developer/MonoTouch/Source/monotouch/src/UIKit/UIApplication.cs:38 at appname.Application.Main (string[]) [0x00000] in /Users/us/Projects/appname/appname/Main.cs:17 at (wrapper runtime-invoke) object.runtime_invoke_dynamic (intptr,intptr,intptr,intptr) <0xffffffff>

Native stacktrace:

0   appname                            0x00a81ba9 mono_handle_native_sigsegv + 244
1   appname                            0x00a48015 mono_sigsegv_signal_handler + 172
2   libsystem_c.dylib                   0x35df3e93 _sigtramp + 42
3   UIKit                               0x33f820a5 <redacted> + 72
4   UIKit                               0x33f82057 <redacted> + 30
5   UIKit                               0x33f82035 <redacted> + 44
6   UIKit                               0x33f818eb <redacted> + 502
7   UIKit                               0x33f81de1 <redacted> + 488
8   UIKit                               0x33ea0421 <redacted> + 5768
9   CoreFoundation                      0x3ad3a6cd <redacted> + 20
10  CoreFoundation                      0x3ad389c1 <redacted> + 276
11  CoreFoundation                      0x3ad38d17 <redacted> + 742
12  CoreFoundation                      0x3acabebd CFRunLoopRunSpecific + 356
13  CoreFoundation                      0x3acabd49 CFRunLoopRunInMode + 104
14  GraphicsServices                    0x390792eb GSEventRunModal + 74
15  UIKit                               0x33eeb2f9 UIApplicationMain + 1120
16  appname                            0x0006f618 wrapper_managed_to_native_MonoTouch_UIKit_UIApplication_UIApplicationMain_int_string___intptr_intptr + 240
17  appname                            0x009bdb20 appname_Application_Main_string__ + 152
18  appname                            0x0068664c wrapper_runtime_invoke_object_runtime_invoke_dynamic_intptr_intptr_intptr_intptr + 200
19  appname                            0x00a4962f mono_jit_runtime_invoke + 1054
20  appname                            0x00aa5df3 mono_runtime_invoke + 90
21  appname                            0x00aa88d7 mono_runtime_exec_main + 306
22  appname                            0x00aabb5b mono_runtime_run_main + 482
23  appname                            0x00a5c133 mono_jit_exec + 94
24  appname                            0x00ae9e3c main + 2220
25  appname                            0x00002028 start + 40

Got a SIGSEGV while executing native code. This usually indicates a fatal error in the mono runtime or one of the native libraries used by your application.

Any ideas what is the error?

Thanks!