fix crash if mProviders is reset while loading
fix crash if mProviders is reset while loading

closes #4166 https://dev.guardianproject.info/issues/4166

Heart2009/ChatSecureAndroid,guardianproject/ChatSecureAndroid,bonashen/ChatSecureAndroid,kden/ChatSecureAndroid,h2ri/ChatSecureAndroid,bonashen/ChatSecureAndroid,guardianproject/ChatSecureAndroid,eighthave/ChatSecureAndroid,kden/ChatSecureAndroid,n8fr8/ChatSecureAndroid,n8fr8/AwesomeApp,anvayarai/my-ChatSecure,joskarthic/chatsecure,eighthave/ChatSecureAndroid,prembasumatary/ChatSecureAndroid,guardianproject/ChatSecureAndroid,n8fr8/ChatSecureAndroid,h2ri/ChatSecureAndroid,anvayarai/my-ChatSecure,prembasumatary/ChatSecureAndroid,joskarthic/chatsecure,anvayarai/my-ChatSecure,joskarthic/chatsecure,bonashen/ChatSecureAndroid,Heart2009/ChatSecureAndroid,prembasumatary/ChatSecureAndroid,Heart2009/ChatSecureAndroid,n8fr8/AwesomeApp,n8fr8/AwesomeApp,n8fr8/ChatSecureAndroid,h2ri/ChatSecureAndroid,eighthave/ChatSecureAndroid,kden/ChatSecureAndroid