Markdown / Editor render problem

Nobody has claimed this yet.

Markdown text doesn't apply to preview pane at all.
#### My Environment

MacBook Air 2011
RAM 4GB
OS Marvericks
OS Language Japanese

When problem occurred, following message logged to "OS's Console App" which is in /Application/Utilities/ and freeze or crash.

2015/02/20 1:51:30.974 MacDown[2807]: *** setObjectForKey: object cannot be nil (key: url)

2015/02/20 1:51:30.976 MacDown[2807]: (

    0   CoreFoundation                      0x00007fff87c7625c __exceptionPreprocess + 172

    1   libobjc.A.dylib                     0x00007fff8d1ace75 objc_exception_throw + 43

    2   CoreFoundation                      0x00007fff87b7122e -[__NSDictionaryM setObject:forKey:] + 1102

    3   MacDown                             0x000000010236013c MacDown + 45372

    4   MacDown                             0x0000000102363e6d MacDown + 61037

    5   MacDown                             0x0000000102363c2e MacDown + 60462

    6   MacDown                             0x000000010236397f MacDown + 59775

    7   Foundation                          0x00007fff84745714 __NSFireTimer + 96

    8   CoreFoundation                      0x00007fff87bdd3e4 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ + 20

    9   CoreFoundation                      0x00007fff87bdcf1f __CFRunLoopDoTimer + 1151

    10  CoreFoundation                      0x00007fff87c4e5aa __CFRunLoopDoTimers + 298

    11  CoreFoundation                      0x00007fff87b986a5 __CFRunLoopRun + 1525

    12  CoreFoundation                      0x00007fff87b97e75 CFRunLoopRunSpecific + 309

    13  HIToolbox                           0x00007fff8e532a0d RunCurrentEventLoopInMode + 226

    14  HIToolbox                           0x00007fff8e5327b7 ReceiveNextEventCommon + 479

    15  HIToolbox                           0x00007fff8e5325bc _BlockUntilNextEventMatchingListInModeWithFilter + 65

    16  AppKit                              0x00007fff86fca24e _DPSNextEvent + 1434

    17  AppKit                              0x00007fff86fc989b -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 122

    18  AppKit                              0x00007fff86fbd99c -[NSApplication run] + 553

    19  AppKit                              0x00007fff86fa8783 NSApplicationMain + 940

    20  libdyld.dylib                       0x00007fff81a655fd start + 1

    21  ???                                 0x0000000000000001 0x0 + 1

)

No source files or tests are named. Start by reproducing the preview-pane failure on macOS using the reported Markdown text and environment, then inspect the Console message about a nil URL. Done means Markdown renders in the preview pane without freezing or crashing.

Written by the indexing model from the issue text.