Guard against null exception (#10162)

lib/system/jssys.nim

@@ -46,7 +46,7 @@ proc nimCharToStr(x: char): string {.compilerproc.} =
   result[0] = x
 
 proc isNimException(): bool {.asmNoStackFrame.} =
-  asm "return `lastJSError`.m_type;"
+  asm "return `lastJSError` && `lastJSError`.m_type;"
 
 proc getCurrentException*(): ref Exception {.compilerRtl, benign.} =
   if isNimException(): result = cast[ref Exception](lastJSError)

tests/js/test2.nim

@@ -9,6 +9,9 @@ js 3.14
 
 # This file tests the JavaScript generator
 
+doAssert getCurrentException() == nil
+doAssert getCurrentExceptionMsg() == ""
+
 #  #335
 proc foo() =
   var bar = "foo"