Fix some possible clipboard crashes on Android

Fixes #1511

CHANGELOG.md

@@ -14,6 +14,7 @@ More detailed release notes can be found on the [releases page](https://github.c
 * iOS compile failed with Go 1.15 (#1497)
 * Possible crash when minimising app containing List on Windows
 * File chooser dialog ignores drive Z (#1513)
+* Entry copy/paste is crashing on android 7.1 (#1511)
 
 
 ## 1.4 - 1 November 2020

internal/driver/gomobile/android.c

@@ -68,19 +68,30 @@ jobject getClipboard(uintptr_t jni_env, uintptr_t ctx) {
 	jmethodID getSystemService = find_method(env, ctxClass, "getSystemService", "(Ljava/lang/String;)Ljava/lang/Object;");
 
 	jstring service = (*env)->NewStringUTF(env, "clipboard");
-	return (jobject)(*env)->CallObjectMethod(env, ctx, getSystemService, service);
+	jobject ret = (jobject)(*env)->CallObjectMethod(env, ctx, getSystemService, service);
+	jthrowable err = (*env)->ExceptionOccurred(env);
+
+	if (err != NULL) {
+		LOG_FATAL("cannot lookup clipboard");
+		(*env)->ExceptionClear(env);
+		return NULL;
+	}
+	return ret;
 }
 
 char *getClipboardContent(uintptr_t java_vm, uintptr_t jni_env, uintptr_t ctx) {
 	JNIEnv *env = (JNIEnv*)jni_env;
 	jobject mgr = getClipboard(jni_env, ctx);
+	if (mgr == NULL) {
+		return NULL;
+	}
 
 	jclass mgrClass = (*env)->GetObjectClass(env, mgr);
 	jmethodID getText = find_method(env, mgrClass, "getText", "()Ljava/lang/CharSequence;");
 
 	jobject content = (jstring)(*env)->CallObjectMethod(env, mgr, getText);
 	if (content == NULL) {
-		return "";
+		return NULL;
 	}
 
 	jclass clzCharSequence = (*env)->GetObjectClass(env, content);
@@ -93,6 +104,9 @@ char *getClipboardContent(uintptr_t java_vm, uintptr_t jni_env, uintptr_t ctx) {
 void setClipboardContent(uintptr_t java_vm, uintptr_t jni_env, uintptr_t ctx, char *content) {
 	JNIEnv *env = (JNIEnv*)jni_env;
 	jobject mgr = getClipboard(jni_env, ctx);
+	if (mgr == NULL) {
+		return;
+	}
 
 	jclass mgrClass = (*env)->GetObjectClass(env, mgr);
 	jmethodID setText = find_method(env, mgrClass, "setText", "(Ljava/lang/CharSequence;)V");

internal/driver/gomobile/clipboard_android.go

@@ -22,6 +22,10 @@ func (c *mobileClipboard) Content() string {
 	content := ""
 	app.RunOnJVM(func(vm, env, ctx uintptr) error {
 		chars := C.getClipboardContent(C.uintptr_t(vm), C.uintptr_t(env), C.uintptr_t(ctx))
+		if chars == nil {
+			return nil
+		}
+
 		content = C.GoString(chars)
 		C.free(unsafe.Pointer(chars))
 		return nil