崩溃访问静态变量 UIFont

Question

快速申请。我声明了一个主题类，包括以下内容：

class Theme {    
    enum Font {
        static let header1         = UIFont(name: "IBM Plex Sans Bold", size: 32)! 
        static let header2         = UIFont(name: "IBM Plex Sans Bold", size: 20)! // Crash
        static let pickerTitleFont = UIFont(name: "IBM Plex Sans Bold", size: 17)!
        static let navBarFont      = UIFont(name: "IBM Plex Sans Bold", size: 28)!  // Crash
    }
}

在appDelegate中，我有一个设置导航栏外观代理的例程，

        let appearance = UINavigationBarAppearance()
        appearance.titleTextAttributes = [NSAttributedString.Key.foregroundColor: UIColor.white,
                                          NSAttributedString.Key.font: Theme.Font.navBarFont]

有时（很少），当运行这个应用程序时，会发生崩溃。

Exception Type:  EXC_BREAKPOINT (SIGTRAP)
Exception Codes: 0x0000000000000001, 0x0000000104d5a7c0
Termination Reason: SIGNAL 5 Trace/BPT trap: 5
Terminating Process: exc handler [19751]

Triggered by Thread:  0


Kernel Triage:
VM - pmap_enter retried due to resource shortage
VM - pmap_enter retried due to resource shortage
VM - pmap_enter retried due to resource shortage
VM - pmap_enter retried due to resource shortage
VM - pmap_enter retried due to resource shortage


Thread 0 name:
Thread 0 Crashed:
0   ---                             0x0000000104d5a7c0 one-time initialization function for pickerTitleFont + 140
1   ---                             0x0000000104d5a7a4 one-time initialization function for pickerTitleFont + 112
2   libdispatch.dylib               0x00000001b8550f88 _dispatch_client_callout + 20 (object.m:560)
3   libdispatch.dylib               0x00000001b85527d4 _dispatch_once_callout + 32 (once.c:52)
4   ---                             0x0000000104d77104 Theme.Font.navBarFont.unsafeMutableAddressor + 20 (Theme.swift:75)
5   ---                             0x0000000104d77104 AppDelegate.setupNavigationController(root:) + 1188 (AppDelegate.swift:152)

这次崩溃怎么会发生？它只是访问一个静态定义的变量。 崩溃没有明确说明崩溃发生的原因。

很奇怪，崩溃似乎是由访问

Font.navBarFont

引起的，但是在堆栈跟踪的较低位置它说它是

pickerTitleFont

.

访问

header2

字体在视图中的标签上设置字体时也会发生此崩溃。