The value NIL is not of type STUMPWM::SCREEN.
0: (SB-DEBUG::MAP-BACKTRACE #<CLOSURE (LAMBDA #) {AFDD6B5}>)[:EXTERNAL]
1: (SB-DEBUG:BACKTRACE 100 #<SB-IMPL::STRING-OUTPUT-STREAM {AFDD661}>)
2: (STUMPWM::BACKTRACE-STRING)
3: ((FLET #:LAMBDA221) #<TYPE-ERROR {AFDD411}>)
4: (SIGNAL #<TYPE-ERROR {AFDD411}>)[:EXTERNAL]
5: (ERROR TYPE-ERROR)[:EXTERNAL]
6: (SB-KERNEL::OBJECT-NOT-TYPE-ERROR-HANDLER
    #<unavailable argument>
    #.(SB-SYS:INT-SAP #XB79D54F0)
    #<SB-ALIEN-INTERNALS:ALIEN-VALUE :SAP #XB79D51DC :TYPE (*
                                                            (SB-ALIEN:STRUCT
                                                             SB-VM::OS-CONTEXT-T-STRUCT))>
    (398 14))
7: (SB-KERNEL::OBJECT-NOT-TYPE-ERROR-HANDLER
    #<unavailable argument>
    #.(SB-SYS:INT-SAP #XB79D54F0)
    #<SB-ALIEN-INTERNALS:ALIEN-VALUE :SAP #XB79D51DC :TYPE (*
                                                            (SB-ALIEN:STRUCT
                                                             SB-VM::OS-CONTEXT-T-STRUCT))>
    (398 14))[:EXTERNAL]
8: (SB-KERNEL:INTERNAL-ERROR
    #.(SB-SYS:INT-SAP #XB79D51DC)
    #<unavailable argument>)
9: ("foreign function: #x806401C")
10: ("foreign function: #x805226C")
11: ("foreign function: #x8056ACF")
12: ("foreign function: #x8056D03")
13: (STUMPWM::SCREEN-ADD-MAPPED-WINDOW NIL #<XLIB:WINDOW :0 2E00D10>)
14: (STUMPWM::ADD-WINDOW NIL #<XLIB:WINDOW :0 2E00D10>)
15: (STUMPWM::PROCESS-MAPPED-WINDOW NIL #<XLIB:WINDOW :0 2E00D10>)
16: ((LABELS #:G145)
     :PARENT
     #<XLIB:WINDOW :0 1A6>
     :SEND-EVENT-P
     NIL
     :WINDOW
     #<XLIB:WINDOW :0 2E00D10>)
17: (STUMPWM::HANDLE-EVENT)[:EXTERNAL]
18: ((FLET SB-THREAD::WITH-RECURSIVE-LOCK-THUNK))
19: ((FLET #:WITHOUT-INTERRUPTS-BODY-[CALL-WITH-RECURSIVE-LOCK]508))
20: (SB-THREAD::CALL-WITH-RECURSIVE-LOCK
     #<CLOSURE (FLET SB-THREAD::WITH-RECURSIVE-LOCK-THUNK) {B79D586D}>
     #S(SB-THREAD:MUTEX
        :NAME "CLX Event Lock"
        :%OWNER #<SB-THREAD:THREAD "initial thread" RUNNING {A6DFBF1}>
        :STATE 1))
21: (XLIB:PROCESS-EVENT #<XLIB:DISPLAY :0 (The X.Org Foundation R10400090)>)[:EXTERNAL]
22: (STUMPWM::STUMPWM-INTERNAL-LOOP)
23: (STUMPWM::STUMPWM-INTERNAL-LOOP)[:EXTERNAL]
24: (STUMPWM::STUMPWM-INTERNAL ":0.0")
25: (STUMPWM ":0.0")
26: (SB-INT:SIMPLE-EVAL-IN-LEXENV (STUMPWM) #<NULL-LEXENV>)
27: (SB-FASL::LOAD-AS-SOURCE
     #<SB-SYS:FD-STREAM for "file /home/rohan/bin/stumpwm" {A94D0A9}>
     NIL
     NIL)
28: ((FLET SB-FASL::LOAD-STREAM)
     #<SB-SYS:FD-STREAM for "file /home/rohan/bin/stumpwm" {A94D0A9}>)
29: (LOAD #P"/home/rohan/bin/stumpwm")[:EXTERNAL]
30: (SB-INT:SIMPLE-EVAL-IN-LEXENV
     (LOAD #P"/home/rohan/bin/stumpwm")
     #<NULL-LEXENV>)
31: (SB-IMPL::PROCESS-EVAL-OPTIONS
     ("(require :asdf)"
      "(setq asdf:*central-registry* '(#p\"/home/rohan/src/clbuild/systems/\"))"
      "nil" (LOAD #P"/home/rohan/bin/stumpwm")))
32: (SB-IMPL::TOPLEVEL-INIT)
33: ((LABELS SB-IMPL::RESTART-LISP))