demo crash when `show a toast` if switch to launcher an other app

[zzhouj]

test case:

1. launcher legend demo app
2. return to home & luacher an other app (eg: calcualtor)
3. return to legend demo & click show a toast button

crash logcat:

--------- beginning of crash
04-02 16:56:27.181 18323 18323 F libc    : Fatal signal 11 (SIGSEGV), code 2, fault addr 0x12c0e084 in tid 18323 (com.legend.demo)
04-02 16:56:27.193   202   749 D audio_hw_primary: out_set_parameters: enter: usecase(1: low-latency-playback) kvpairs: routing=2
04-02 16:56:27.282   199   199 F DEBUG   : *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
04-02 16:56:27.282   199   199 F DEBUG   : Build fingerprint: 'Android/aosp_hammerhead/hammerhead:6.0/MRA58K/potato03232147:userdebug/test-keys'
04-02 16:56:27.282   199   199 F DEBUG   : Revision: '0'
04-02 16:56:27.282   199   199 F DEBUG   : ABI: 'arm'
04-02 16:56:27.282   199   199 F DEBUG   : pid: 18323, tid: 18323, name: com.legend.demo  >>> com.legend.demo <<<
04-02 16:56:27.282   199   199 F DEBUG   : signal 11 (SIGSEGV), code 2 (SEGV_ACCERR), fault addr 0x12c0e084
04-02 16:56:27.302   199   199 F DEBUG   :     r0 b5985000  r1 ffffffff  r2 00000001  r3 00000001
04-02 16:56:27.303   199   199 F DEBUG   :     r4 be8fd0ec  r5 b5985000  r6 be8fd124  r7 be8fd008
04-02 16:56:27.303   199   199 F DEBUG   :     r8 b6d31ec0  r9 be8fd120  sl 12c0e000  fp 00000000
04-02 16:56:27.303   199   199 F DEBUG   :     ip b4b3b085  sp be8fd000  lr b4b84db9  pc b4b84dc2  cpsr 00070030
04-02 16:56:27.311   199   199 F DEBUG   : 
04-02 16:56:27.311   199   199 F DEBUG   : backtrace:
04-02 16:56:27.311   199   199 F DEBUG   :     #00 pc 0032ddc2  /system/lib/libart.so (art::InvokeMethod(art::ScopedObjectAccessAlreadyRunnable const&, _jobject*, _jobject*, _jobject*, unsigned int)+97)
04-02 16:56:27.311   199   199 F DEBUG   :     #01 pc 002e40a5  /system/lib/libart.so (art::Method_invoke(_JNIEnv*, _jobject*, _jobject*, _jobject*)+32)
04-02 16:56:27.311   199   199 F DEBUG   :     #02 pc 71ef0e39  /data/dalvik-cache/arm/system@[email protected] (offset 0x1eb5000)
04-02 16:56:27.693   199   199 F DEBUG   : 
04-02 16:56:27.693   199   199 F DEBUG   : Tombstone written to: /data/tombstones/tombstone_09
04-02 16:56:27.693   199   199 E DEBUG   : AM write failed: Broken pipe