WaveformArea: fixed broken null check causing segfault when attempting to autoscale a null waveform. Fixes #978.

Author: azonenberg

release-notes/CHANGELOG.md

@@ -93,6 +93,7 @@ NOTE: This section only list changes which are potentially breaking to an *end u
 * GUI: Pressing middle mouse on the Y axis to autoscale would fail, setting the full scale range to zero volts, if the waveform was resident in GPU memory and the CPU-side copy of the buffer was stale
 * GUI: History dialog allowed zero or negative values for history depth (https://github.com/ngscopeclient/scopehal-apps/issues/940)
 * GUI: Eye patterns and constellations would forget the selected color ramp when moved to a new location (https://github.com/ngscopeclient/scopehal-apps/issues/556)
+* GUI: crash when autoscaling an empty waveform (https://github.com/ngscopeclient/scopehal-apps/issues/978)
 * Session files: Windows build could not load session files containing sample rates or memory depths in excess of 2^32
 
 ## Other changes since v0.1.1

src/ngscopeclient/WaveformArea.cpp

@@ -4524,6 +4524,8 @@ void WaveformArea::AutofitVertical()
 	for(auto& c : m_displayedChannels)
 	{
 		auto data = c->GetStream().GetData();
+		if(!data)
+			continue;
 		data->PrepareForCpuAccess();
 		auto sdata = dynamic_cast<SparseAnalogWaveform*>(data);
 		auto udata = dynamic_cast<UniformAnalogWaveform*>(data);