游乐场执行失败：错误：无法使用swift查找符号 – xcode中的Playground

我刚刚开始使用playground并尝试运行一些基本代码，例如：

import UIKit var str = "Hello, playground" 

我想它应该是实时运行但是在我的情况下运行需要永远，并且在一段时间后，我得到以下错误：

 Playground execution failed: error: Couldn't lookup symbols: __swift_FORCE_LOAD_$_swiftCoreGraphics __swift_FORCE_LOAD_$_swiftUIKit __swift_FORCE_LOAD_$_swiftDarwin __swift_FORCE_LOAD_$_swiftCoreImage __swift_FORCE_LOAD_$_swiftFoundation __swift_FORCE_LOAD_$_swiftObjectiveC _playground_log_hidden __swift_FORCE_LOAD_$_swiftDispatch _playground_logger_initialize _playground_log_postprint thread #1: tid = 0x39745, 0x00000001006283c0 MyPlayground2`executePlayground, queue = 'com.apple.main-thread', stop reason = breakpoint 1.2 * frame #0: 0x00000001006283c0 MyPlayground2`executePlayground frame #1: 0x00000001006279c0 MyPlayground2`__37-[XCPAppDelegate enqueueRunLoopBlock]_block_invoke + 32 frame #2: 0x000000010114225c CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12 frame #3: 0x0000000101127304 CoreFoundation`__CFRunLoopDoBlocks + 356 frame #4: 0x0000000101126a75 CoreFoundation`__CFRunLoopRun + 901 frame #5: 0x0000000101126494 CoreFoundation`CFRunLoopRunSpecific + 420 frame #6: 0x0000000106531a6f GraphicsServices`GSEventRunModal + 161 frame #7: 0x0000000101cd0f34 UIKit`UIApplicationMain + 159 frame #8: 0x00000001006276e9 MyPlayground2`main + 201 frame #9: 0x000000010467e68d libdyld.dylib`start + 1